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i

NASA/TP—2011–216456

Design of Refractory Metal Heat Pipe Life

Test Environment Chamber, Cooling System, 

and Radio Frequency Heating System

J.J. Martin, S.M. Bragg-Sitton, R.S. Reid, E.T. Stewart, and J.D. Davis

Marshall Space Flight Center, Marshall Space Flight Center, Alabama

February 2011

National Aeronautics and

Space Administration

Marshall Space Flight Center • MSFC, Alabama 35812
ii

Available from:

NASA Center for AeroSpace Information

7115 Standard Drive

Hanover, MD 21076 –1320

443–757–5802

This report is also available in electronic form at

<https://www2.sti.nasa.gov>

TRADEMARKS

Trade names and trademarks are used in this report for identification only. This usage does not constitute an official

endorsement, either expressed or implied, by the National Aeronautics and Space Administration.
iii

TABLE OF CONTENTS

1. INTRODUCTION ........................................................................................................................... 1

2. ENVIRONMENTAL TEST CHAMBERS ..................................................................................... 3

2.1 General Layout and Components ............................................................................................ 3

2.2 Engineering Design ................................................................................................................. 6

2.3 General Procurement Specifications ........................................................................................ 6

3. WATER COOLING ......................................................................................................................... 8

3.1 General Requirements Layout Description: Calorimeter Assemblies ..................................... 8

3.2 Coolant Water Quality ............................................................................................................. 9

3.3 Sizing and Performance ........................................................................................................... 12

3.4 General Fill and Test Procedures ............................................................................................. 19

3.5 General Procurement Specifications ........................................................................................ 23

4. RADIO FREQUENCY HEATING SYSTEM ................................................................................ 24

4.1 General Heat Pipe Test Arrangement ...................................................................................... 24

4.2 General Inductive Coil Geometry ............................................................................................ 26

4.3 Inductive Coil Sizing and Performance Parameters ................................................................ 28

4.4 Overall Radio Frequency Power System Layout .................................................................... 38

4.5 General Procurement Specifications ........................................................................................ 40

5. PURIFIED INERT GAS CONDITIONING SYSTEM .................................................................. 42

5.1 Sizing and Performance ........................................................................................................... 42

5.2 General Procurement Specifications ........................................................................................ 56

6. INSTRUMENTATION ................................................................................................................... 58

6.1 Data Acquisition and Control Methodology ............................................................................ 58

6.2 Instrumentation Components and Procurement Specifications ............................................... 61

6.3 Data Quality ............................................................................................................................. 65

7. SUMMARY ..................................................................................................................................... 67

APPENDIX A—ENVIRONMENTAL TEST CHAMBER ENGINEERING DRAWINGS .............. 68
iv

TABLE OF CONTENTS (Continued)

APPENDIX B—HEAT PIPE CLUSTER SUPPORT ENGINEERING DRAWINGS ....................... 71

APPENDIX C—CALORIMETER ENGINEERING DRAWINGS ................................................... 74

APPENDIX D—ENVIRONMENTAL TEST CHAMBER PROCUREMENTS ............................... 76

APPENDIX E—CALORIMETER COOLING SUBSYSTEM PROCUREMENTS ......................... 77

APPENDIX F—RADIO FREQUENCY INDUCTIVE COIL THERMAL 

LOAD WORKSHEET .............................................................................................. 78

APPENDIX G—MOLYBDENUM-RHENIUM AND SODIUM RESISTIVITY PROPERTIES ...... 80

APPENDIX H—RADIO FREQUENCY INDUCTIVE COIL ONE-DIMENSIONAL 

SIMULATION RESULTS ........................................................................................ 82

APPENDIX I— RADIO FREQUENCY INDUCTIVE COIL TWO-DIMENSIONAL 

SIMULATION RESULTS ........................................................................................ 84

APPENDIX J—RADIO FREQUENCY INDUCTIVE COIL ASSEMBLY DESIGN

AND FABRICATION WITH ASSOCIATED BUSSWORK 

SPECIFICATIONS .................................................................................................... 85

J.1 Integrated Financial Management Program Procurement ..................................................... 85

J.2 Radio Frequency Coil and Network Specifications .............................................................. 86

APPENDIX K—GAS PURIFICATION SUBSYSTEM PROCUREMENTS .................................... 90

APPENDIX L—WATER-COOLING SUBSYSTEM INSTRUMENTATION ................................... 92

APPENDIX M—MASS AND VOLUMETRIC FLOW METERS FOR LOW-PRESSURE

HELIUM/ARGON MIXTURES ............................................................................. 95

M.1 Thermal Mass Flow-Type Meters ......................................................................................... 95

M.2 Laminar Volumetric-Type Flow Meters ................................................................................ 96

APPENDIX N—TWO-BAND OPTICAL PYROMETER SPECIFICATIONS ................................. 97

APPENDIX O—VACUUM GAUGING SPECIFICATIONS ............................................................. 98

APPENDIX P—DISTRIBUTED FIELDPOINT INPUT/OUTPUT COMPONENT 

SPECIFICATIONS ................................................................................................... 99

REFERENCES .................................................................................................................................... 100
v

LIST OF FIGURES

 1. General configuration of large environmental test chambers .............................................. 4

 2. Heat pipe and calorimeter mounting support structure ........................................................ 5

 3. Heat pipe and calorimeter attachment bracket ..................................................................... 5

 4. General calorimeter water-cooling system layout (one calorimeter illustrated) .................. 9

 5. Computed system pressure-flow curves for water-cooling system ...................................... 18

 6. General test chamber layout showing the typical five-position RF coil cluster .................. 25

 7. Sample RF inductive coil layout .......................................................................................... 27

 8. General test chamber section/RF inductive coil configuration ............................................ 27

 9. Direct current breakdown voltages in pure He, Ar, and He-Ar mixtures 

(Paschen curves) .................................................................................................................. 28

10. Breakdown voltage range with a total operating pressure of 76 torr ................................... 29

11. Radio frequency inductor to heat pipe geometry ................................................................. 30

12. Heat pipe to RF inductive coil heat loss versus standoff distance (pressure = 76 torr) ....... 31

13. Radio frequency inductive coil cluster (five coils) power loss due to internal 

RF losses and thermal transport from heat pipe ................................................................... 32

14. Radio frequency inductive coil cluster (five coils) required coolant water flow rate .......... 33

15. One-dimensional simulation results for inductive coil efficiency ....................................... 34

16. One-dimensional simulation results RF power supply output requirement (five coils) ...... 35

17. One-dimensional simulation results—coil assembly required voltage (five coils) ............. 35

18. Radio frequency coil with flux concentrator in open space ................................................. 37

19. Radio frequency coil without flux concentrator in open space ............................................ 37
vi

LIST OF FIGURES (Continued)

20. Radio frequency coil with flux concentrator; close to neighboring inductor ....................... 38

21. Radio frequency coil without flux concentrator; close to neighbor inductor ....................... 38

22. General RF power subsystem hardware layout ................................................................... 39

23. Oxygen concentration (ppmv) in the heat pipe test chambers following

successive dilutions with (a) UHP He or (b) UHP Ar fill gas .............................................. 43

24. Test chamber gas mixture and purification system .............................................................. 44

25. Final oxygen concentration in the heat pipe test chambers with UHP gas, 

purified using an inline SAES microtorr ambient gas purifier to 1 ppb O2

prior to chamber inlet ........................................................................................................... 45

26. Simplified test loop for pressure drop calculation ............................................................... 48

27. Residual gas analyzer sampling system ............................................................................... 52

28. General data acquisition and control system subsystems .................................................... 60

29. Test chamber assembly (three segments integrated) (Dwg. No. 90M11906) ...................... 68

30. Detail test chamber shell (Dwg. No. 90M11906) ................................................................ 69

31. Detail test chamber support structure (Dwg. No. 90M11906) ............................................. 70

32. Heat pipe and calorimeter cluster attachment support and bracket assembly 

(Dwg. No. 90M11911) ......................................................................................................... 71

33. Attachment support structure details (Dwg. No. 90M11911) .............................................. 72

34. Heat pipe and calorimeter mounting bracket detail (Dwg. No. 90M11911) ........................ 73

35. Heat pipe calorimeter assembly (Dwg. No. 90M11898) ..................................................... 74

36. Calorimeter design detail (Dwg. No. 90M11898) ............................................................... 75

37. Electrical resistivity curves for Na and Mo-50%Re ............................................................ 81

38. General dimensional layout of a single RF inductive coil position ..................................... 86

39. Direct current breakdown voltages in pure He, Ar, and mixtures (Paschen curves) ........... 88
vii

LIST OF TABLES

 1. Baseline heat pipe life test matrix conditions ...................................................................... 2

 2. Water grades from the ASTM standard specification for reagent water ............................. 11

 3. Comparisons for water and solutions of water and DOWTHERM 4000 

(concentrations given in percent volume) for F(-4) (all properties assumed 

constant) ............................................................................................................................... 11

 4. Flow element descriptions for the water-cooling system spreadsheet model ...................... 13

 5. Key parameters for the calorimeter section spreadsheet model for test case F(-4) ............. 14

 6. Calculated pressure drops for each of the 16 heat pipe test cases ....................................... 16

 7. Key parameters for the calorimeter and supply section spreadsheet model ........................ 17

 8. Proposed heat pipe life test matrix ....................................................................................... 24

 9. Position of heat pipes within test chamber clusters ............................................................. 26

10. SAES monotorr phase II 3000 performance guarantee for rare gases ................................. 46

11. Loss coefficients for system pressure drop calculation ........................................................ 48

12. Approximate outgassing rates of stainless steel as reported in Roth ................................... 49

13. Approximate volume and mass of water vapor emanating from the three large 

test chamber surfaces per unit time ...................................................................................... 50

14. Thermocouple to pyrometer comparison ............................................................................. 63

15. General hardware needed for fabrication of environmental test chambers ......................... 76

16. General hardware list for assembly of the heat pipe water-cooling subsystem ................... 77

17. Heat transfer loss estimates between RF inductive coil and heat pipe surface .................... 79

18. Sodium resistivity ................................................................................................................ 80

19. Mo-50%Re resistivity .......................................................................................................... 80
viii

LIST OF TABLES (Continued)

20. Radio frequency inductive coil one-dimensional simulation results using 

one- and five-coil calculations ............................................................................................. 83

21. Radio frequency inductive coil two-dimensional simulation results using 

one- and five-coil calculations ............................................................................................. 84

22. Required RF power induced in the part without thermal losses .......................................... 87

23. General hardware for gas purification subsystems (large/small chambers) ......................... 91

24. General instrumentation options for water-cooling system ................................................. 92

25. Optical pyrometer specifications .......................................................................................... 97

26. Kurt J. Lesker multigauge box system with control boards ................................................. 98

27. Vacuum pressure sensors for the multigauge box ................................................................ 98

28. FieldPoint I/O components .................................................................................................. 99
ix

LIST OF ACRONYMS AND SYMBOLS

AI aperture inlet

Ar argon

ASME American Society of Mechanical Engineers

ASTM American Society for Testing and Materials

BPVC Boiler and Pressure Vessel Code

CH4 methane

CO carbon monoxide

CO2 carbon dioxide

CTI capillary tube inlet

DC direct current

DI deionized

DO dissolved oxygen

ELTA Electro-Thermal Analysis (marketed by BNV Corp) 

FP FieldPoint

GFSSP Generalized Fluid System Simulation Program

H2 hydrogen

H2O water

HCI high-conductance inlet

He helium

IAPWS International Association for the Properties of Water and Steam

ID inside diameter

I/O inlet/outlet

Mo molybdenum

MSFC Marshall Space Flight Center

N nitrogen

N2 nitrogen
x

LIST OF ACRONYMS AND SYMBOLS (Continued)

Na sodium

NDE nondestructive evaluation

NI National Instruments

NIST National Institute of Standards and Technology 

O2 oxygen

OD outside diameter

PLC programmable logic controller

Re rhenium

RF radio frequency

RGA residual gas analyzer

RT real time

RTC real-time controller

SA surface area

SS stainless steel

TBD to be determined

TC thermocouple

TP Technical Publication

UHP ultra-high purity

UV ultraviolet

VCR type of metal seal
xi

NOMENCLATURE

A area

Ac cross-sectional area normal to flow (m2)

Ai surface area inner wall (m2)

Ao surface area outer wall (m2)

B test chamber acceptable operating stress (psi)

cp specific heat (kJ/kg-K)

D diameter of cooling system piping outlet manifold to inlet manifold (m)

Do test chamber diameter (in)

d outer (larger) diameter of an annular gap (m)

dh hydraulic diameter (m)

di inner (smaller) diameter of an annular gap (m)

do outer (larger) diameter of an annular gap (m)

f friction factor

f

T friction factor in zone of complete turbulence

Gr Grashof number

g gravity (m/s2)

H	 height (m)

h	 convective heat transfer coefficient

hl head loss (m2/s2)

I	 current (amps)

ID inside diameter (m)

Igap inductive coil as a function of the gap width

K resistance coefficient – K=fL/dh

k effective thermal conductivity (W/m-K)

L length (m)
xii

NOMENCLATURE (Continued)

Lc inductor length (cm)

Le equivalent length

m mass flow rate (kg/s)

m coolant RF inductor system mass flow rate (kg/s)

m tot total mass flow rate through the water cooling system (kg/s)

n viscosity correction exponent

Nu Nusselt number

P pressure (MPa)

P1 static pressure at the inlet

P2 static pressure at the outlet

Pa test chamber pressure (psi)

Pr Prandtl number

p pressure (Pa)

Q flow rate (m3/s)

Q&

cond conductive heat transfer (W)

Q&

losses total power absorbed by inductive coil (thermal and electrical) (W)

Q&

rad radiation heat transfer (W)

Q&

total total heat transfer (W)

qD outgassing rate (torr-l/s-cm2)

Ra Rayleigh number

Re Reynolds number – Re= ρVdh/μ

Re* critical Reynolds number for annular friction factor correlation

r	 radius of the restriction

T temperature (K)

Tin coolant inlet temperature (K)

Tind inductor temperature (K)

Tout coolant outlet temperature (K)
xiii

NOMENCLATURE (Continued)

Thp heat pipe temperature (K)

Ts surface temperature (K)

Tw wall temperature (K)

T∞ fluid temperature far from wall (K)

t test chamber wall thickness (in)

V volume (m3); voltage

Vh dynamic pressure – Vh= ρV 2/2 (MPa)

v mean velocity of flow (m/s)

β coefficient of expansion

∆H change in elevation (m)

∆P change in pressure (MPa)

Γ viscosity ratio – Γ =μb /μw

ε pipe diameter ratio – di /do; emissivity

εi inner surface emissivity

εο outer surface emissivity

μ absolute (dynamic) viscosity (Pa-s or kg/m-s)

μb absolute (dynamic) viscosity at the mean fluid bulk temperature (Pa-s or kg/m-s)

μw absolute (dynamic) viscosity at the mean wall temp (Pa-s or kg/m-s)

ρ density (kg/m3)

σ Stefan-Boltzmann constant (W/m2-K4)

η absolute viscosity of the fluid (kg/m-s)

ψ annular friction factor correction
xiv
1

TECHNICAL PUBLICATION

DESIGN OF REFRACTORY METAL HEAT PIPE LIFE TEST ENVIRONMENT CHAMBER, 

COOLING SYSTEM, AND RADIO FREQUENCY HEATING SYSTEM

1. INTRODUCTION

A proposed approach to evaluate molybdenum- (Mo-) 44.5% rhenium (Re) alloy/sodium (Na) heat 

pipes for life-limiting effects was reported by Martin et al.1 The baseline test matrix includes 16 heat pipes 

(12 in long with a diameter of 5/8 in) operated at various power levels, temperatures, and durations. Tests 

are planned to proceed around the clock with removal, examination, and test resumption at 6-mo intervals. A combination of nondestructive and destructive evaluations will be made to determine aging effects 

within the heat pipe during removal. Heat pipe testing will include a series following the guidelines specified in American Society for Testing and Materials (ASTM) G68-80 (referred to as the G-series) in which 

all heat pipes will be operated at the same temperature and power but for variable operational exposure to 

control the total mass fluence.2 A second set of tests based on a central composite structure test design will 

be evaluated to examine cross correlations between mass fluence, temperature, and power.3 A summary 

of the baseline test conditions for this evaluation is provided in table 1. The required test conditions are 

achieved with a series of subsystems, including the following:

• Environmental test chambers—Provide containment and mounting support for the heat pipes; flooded 

with a purified inert gas atmosphere (helium (He)/argon (Ar) mixture).

• Gas conditioning—Provides an actively purified inert He/Ar mixture to conductively couple the heat 

pipe condenser and calorimeter by means of a gas gap.

• Radio frequency (RF) power/inductive coils—Provide a clean, noncontact and reliable method of imposing a well-defined mass transfer boundary at the heat pipe evaporator.

• Water cooling—Provides primary method for monitoring heat pipe throughput with a condenser-mounted 

calorimeter.

Typical operations and procedures to govern the expected use of the hardware subsystems have 

been outlined and will serve as a guide during buildup and checkout testing. It is expected that a number 

of checkout tests will be required to adequately verify that subsystems meet the design goals. These procedures and process descriptions represent a growing document that will be updated with findings accumulated throughout the project.
2

Table 1. Baseline heat pipe life test matrix conditions.

Test 

Identifier

Power

Level

(W)

Heat Pipe 

Temperature 

(K)

Gas Gap 

Mixture

Gas Gap 

Width 

(in)

Coolant

Flow

(gpm)

Coolant Delta 

Temperature 

(K)

Number 

of Heat 

Pipes

G-Series 3,000 1,273 He-32%Ar 0.025 0.61 18.8 7

F(-4) 5,000 1,273 He-6%Ar 0.025 0.77 24.6 1

F(-3) 1,000 1,273 He-32%Ar 0.103 0.11 35.2 1

F(-2) 3,000 1,373 He-32%Ar 0.031 0.56 21.1 1

F(-1) 3,000 1,173 He-32%Ar 0.021 0.65 17.6 1

F(0) 3,000 1,273 He-32%Ar 0.025 0.61 18.8 1

F(1) 2,000 1,223 He-32%Ar 0.037 0.54 13.9 1

F(2) 4,000 1,223 He-32%Ar 0.017 0.67 22.8 1

F(3) 2,000 1,323 He-32%Ar 0.046 0.50 15.2 1

F(4) 4,000 1,323 He-32%Ar 0.020 0.63 24.0 1

Budgetary limitations and material availability allow for comparatively few tests to study a few 

key variables. Tests would employ the best available processing procedures, incorporating contamination 

getters in each pipe. This approach would also provide margins typical of flight heat pipe designs. The heat 

pipe life test matrix considers a relevant range of operating temperatures and evaporator mass fluences. To 

avoid random introduction of external impurities, isolating boundary conditions will be imposed on the 

heat pipes. Life tests that compress operating time may approach boiling limits; therefore, initial performance tests would be required to determine the achievable compression. The proposed test series reflects 

constraints, yet permits the scope to be expanded in a modular fashion comensurate with changing design 

and material system requirements. 
3

2. ENVIRONMENTAL TEST CHAMBERS

The evaluation of 16 heat pipes will require a series of environmental test chambers containing controlled purity, inert gas environments to provide sufficient thermal coupling between the heat 

pipe condenser sections, and water flow calorimeters. The high-purity gas is necessary to avoid random 

introduction of external contamination. Two types of environmental test chambers have been outlined to 

satisfy the test requirement. These include several large chambers, each containing a cluster of five heat 

pipe/calorimeter assemblies (arranged in a pentagonal configuration) and a small test chamber that contains a single, high-power heat pipe. The large test chambers require a more stringent design cycle due to 

their size (account for applied pressure loads); therefore, the primary design emphasis for this effort was 

directed toward the larger test chambers. The shell and flange arrangement on the smaller chamber will 

allow use of commercial off-the-shelf vacuum-rated items. To mount heat pipes and calorimeters, a common mounting bracket and support structure will be used in all test chambers. In general, commonality 

will be maintained (to the extent possible) with respect to the designs, selection of hardware items, and 

overall construction to allow flexibility in the final hardware setup.

2.1 General Layout and Components

To support operations anticipated over the course of this project, the environmental test chambers 

have been designed for flexible use (operated individually or integrated into a single unit). The final design 

is sized with sufficient internal volume to ease access for installation and maintenance and uses similar 

components in all test chambers for commonality. Figure 1 illustrates the general layout with two test 

chambers integrated into a single assembly and the third unit separated—all three share a common gas 

system. The chambers are constructed of stainless steel, which is well suited for operating in both vacuum 

and in low-pressure, high-purity internal environments. The large chambers will have a cylindrical barrel 

diameter of 24 in with a length of 36 in. The chamber diameter was set based on the largest commercial 

off-the-shelf metal seal flange available (27-in diameter). A number of penetrations provide access for RF 

power feeds, water coolant inlet/outlet (I/O) flow, vacuum source, purified circulation gas, thermocouples, 

and optical access for pyrometers. These penetrations will be a combination of 2- and 6-in-diameter tube 

half-nipples, sealed with knife-edge seal flanges (Conflat and VCR type). 

The calorimeter water feed lines present a more challenging situation since the current baseline 

uses copper tubing. Two methods are available to seal these tubes at the test chamber stainless steel 

interface flange. The first requires a copper to stainless steel transition. (The use of mechanical fittings 

on the water lines inside the test chamber is to be avoided at all costs.) Initial investigations indicate that 

these transition pieces—produced by explosive bonding a sheet of stainless steel to a sheet of copper 

and then machining the required fittings—are commercially available. Unfortunately, they have an unacceptably high cost per fitting and vendors require a minimum purchase. The second alternative is to weld 

a stainless steel metal compression ring-type fitting to the chamber flange interface and to extend the 

calorimeter copper feed lines through these fittings. The compression fittings would then be tightened 
4

End View

Only Two-Band Optical Pyrometer 

Ports Shown for Clarity - x5 Ports 

Per Test Segment

Rail Supports

View

Port

(x5)

View

Port

(x5)

View Port (x5)

View Port

View Port Two-Band Optical Pyrometers 

(x5) One Per Heat Pipe

Two-Band Optical Pyrometers 

(x5) One Per Heat Pipe

Purified

Gas

Line

Purified

Inert Gas

Supply

Turbo Pump Unit 

With Isolation Valve

RF Coil Power and 

Support Penetration

RF Coil Power and 

Support Penetration

RF Coil Power and 

Support Penetration

Coolant Supply

Coolant Supply

Environmental Test Chamber

Segment No. 1

Environmental Test Chamber

Segment No. 2

Environmental Test Chamber

Segment No. 3

Figure 1. General configuration of large environmental test chambers.

on the outside of the chamber, biting into the copper feed line to provide a seal against atmospheric pressure. The copper calorimeter feed line that extends beyond the chamber-mounted compression fitting 

(external to the chamber) can now be hooked into the cooling water subsystem. Compression-type fittings 

are typically avoided for vacuum applications because it is difficult to maintain seal consistency; however, 

for this application, there are few cost-effective alternatives. Engineering drawings for the environmental 

test chamber and its support stand are provided in appendix A.

Mechanical support for the heat pipe and calorimeter clusters contained within the large test chambers is provided by a support structure and frame clamped to internal rails. These rails are drilled and 

tapped to provide a number of tie-down locations that allow freedom in positioning and alignment of the 

heat pipes. This support structure is illustrated in figure 2. 

The calorimeter and heat pipe attachment bracket are attached to the internal support structure 

using a tight tolerance concentric arrangement as shown in figure 3. The central bore is machined slightly 

larger than the heat pipe support tube, allowing for insertion of an insulating material for final heat pipe 

centering. The insulator would allow continuity measurements to be made between the heat pipe and 

the calorimeter to monitor for the possibility of contact during testing. The use of an insulator, however, 

introduces a number of issues, such as potential contamination and a loss in dimension tolerance due to 

the difference in materials and ability to machine. Tight dimensional constraints are mandatory due to 

the small gas conduction gap separating the heat pipe and calorimeter. Before pursuing this option in 

earnest, a detailed examination will be performed to justify its need and to identify available alternatives. At present, a metallic insert can be placed at this location to allow the insulator to be incorporated 

later. In the baseline approach, the heat pipe is supported in cantilever fashion at the condenser end of 

the support tube (shown in fig. 3). If it becomes necessary to support the heat pipe from the evaporator end, due to alignment issues or potential vibration, an additional bracket can be mounted on the
5

Support Structure

Calorimeters (x5) Heat Pipes (x5)

Heat Pipes 

Support Structure

RF Inductive

Coils (×5)

Test Chamber

Support Rail

Figure 2. Heat pipe and calorimeter mounting support structure.

Coolant Outlet

Coolant Inlet

Calorimeter Heat Pipe Condenser

End Plug

Heat Pipe Closeout Tube

and Support

Calorimeter Holder

and Alignment Slot

Heat Pipe Alignment Insert

(Metallic or Insulator)

Heat Pipe Calorimeter

Support Structure

Heat Pipe and

Calorimeter Attachment

Bracket

Figure 3. Heat pipe and calorimeter attachment bracket.

support structure’s lower frame that makes contact on the evaporator end plug extension. The engineering drawings for the support structure/attachment bracket and calorimeters are provided in appendices B 

and C.

External test chamber cooling, if needed, would be provided by forced convection generated by 

high-velocity electric fans placed around the chambers. Even though there is significant power dissipated 

into each of the heat pipe clusters, the majority of this power is absorbed by the calorimeter and RF inductor 

water-cooling systems. In the final test configuration, very little of the active heat pipe surface is exposed 

to the test chamber walls. If the test chamber becomes unacceptably hot during test, copper tubing 
6

connected to the facility water system can be wrapped snugly around the chamber external wall, covering 

the affected areas. For flange surfaces, stainless steel tubing can be coiled around the desired region and 

tack welded in place to provide additional cooling.

2.2 Engineering Design

Large environmental test chamber designs are governed by the American Soiciety of Mechanical 

Engineers (ASME) Boiler and Pressure Vessel Code (BPVC).4 ASME code applies for vacuum system 

applications since primary operation of the test chambers will be at significantly reduced pressure. The 

software used in producing engineering drawings is Microstation Version 7.0 (marketed by Bentley Systems). The vacuum chamber design is performed in accordance with the ASME BPVC, section VIII, division I, part UG-28 for thickness of shells and tubes under external pressure. The calculations performed 

for the test chamber are based on the following approach:

(1) Establish initial geometric conditions, including the outside diameter of the pressure shell 

(Do=24 in), shell thickness (t=0.188 in), and an overall length between flanges (L=33.75 in). 

(2) Verify ratio of Do /t>10 → 24.0/0.188=128

(3) Assess ratio of L/Do → 33.75/24.0=1.41

(4) An acceptable operating stress of B=6,800 psi is calculated using the above factors and the 

appropriate figures within the ASME BPVC, subpart III of section II to determine the operating stress with 

the following applied inputs:

• Material properties for stainless steel 316.

• Material average operating temperature (set to 375 oC, conservative estimate).

(5) The maximum allowable external pressure can then be assessed:

Pa=4B/3(Do /t) → Pa=(4*6,800)/(3*(24/0.188))=71.0 psi.

(6) The calculated value of Pa is compared to the actual value of external pressure of 14.7 psi. 

Because the calculated maximum allowable pressure is much greater than 14.7 psi, the selected wall thickness is acceptable per the ASME BPVC. 

All vacuum flanges in the proposed design are standard vacuum components and require no further 

analysis; however, additional analysis can be performed if nonstandard hardware is required. Structural 

loads on the designed chamber external support stand are minimal and standard materials and welding 

practices are sufficient, requiring no additional analysis. 

2.3 General Procurement Specifications

Hardware procurements necessary to complete the fabrication and assembly of the baseline environmental test chamber assembly have been identified. Hardware purchases were planned to be completed 
7

as solicitations issued by Marshall Space Flight Center (MSFC) using full and open competition. The final 

evaluation of vendor quotes will be based on the lowest cost technically acceptable approach. The current 

baseline requires sufficient material to fabricate three large chambers and one small chamber. The primary 

components within the test systems include the following:

(1) Three large chamber cylindrical pressure shells (24-in-diameter pipe segments or rolled plate) 

with electropolished inner surface suitable for vacuum application. These are commercial off-the-shelf 

items; current vendor cost is estimated at $2.5K each with a delivery of 4 to 6 wk.

(2) Six large chamber flange sets (metal wire seal type), including the chamber mounted flange 

(bored) and blank end plate. These flanges are commercial off-the-shelf items; current vendor cost is estimated at $5K per set (bored and blank) with a delivery of 4 to 6 wk.

(3) Small chamber pressure shell with attached flanges (typical 12-in-diameter vacuum fullnipple section). These are commercial off-the-shelf components with an estimated price in the $5K to $7K 

range, available from a variety of vendors with typical delivery of 4 to 6 wk. 

(4) Variety of 2- and 6-in tube, half-nipple flange penetrations with associated blank flanges, optical view ports, fluid feedthroughs, valves, and bolt/nut hardware. These items are available from a number 

of vacuum vendors, with availability estimated at 4 to 6 wk. The cost of these components is expected to 

be on the order of $12K to $15K.

(5) Structural material to fabricate support stands, internal frames, and mounting brackets from 

stainless steel and refractory metals. Also includes mechanical positioners to accurately control placement of the heat pipes and calorimeters. This material is commercial off-the-shelf with cost estimated at 

approximately $15K to $20K and a delivery of 4 to 6 wk.

(6) Part fabrication and assembly at machine shops, as required to complete test chambers. 

On-demand basis with estimated cost in the $10K to $15K range.

Appendix D provides a partial listing of primary components selected for assembly of the environmental test chambers. Additional details and possible substitutions will be added when the design 

is finalized.
8

3. WATER COOLING

To support test operations, water cooling is necessary for several hardware subsystems, including the heat pipe calorimeter assemblies, the environmental test chamber walls, if required, and the RF 

power supplies/induction coils. Each of these is serviced by a closed recirculation loop that rejects waste 

heat to a common facility coolant system using water-water heat exchangers. The sophistication of each 

loop is dependent on materials selection (corrosion potential) and the minimum flow passage size (small 

sizes are more likely to be blocked). The chamber cooling traces are of the lowest concern since they will 

be 0.25-in-diameter tubes or larger located external to the environmental test chamber. Because leakage 

and service present minimal concern for chamber cooling, they can make use of coolant water directly 

from the facility system—no intermediate conditioning system. The RF power supplies and inductive coil 

assemblies are of moderate concern since coil cooling traces penetrate the environmental test chamber and 

must carry a significant amount of waste heat. The coil traces will be constructed from 0.25-in-diameter 

tubing or larger to minimize the risk of plugging. The current baseline is to use a commercial circulation 

unit to process the coolant water for this system.

The calorimeter assemblies serve a critical test role and are of high concern since they are positioned in very close proximity to the heat pipes and extract large amounts of power. The calorimeter makes 

use of a small water flow annulus (width of 0.01 to 0.015 in) to absorb power, placing increased emphasis 

on the control of particulates and corrosion to minimize the potential for blockage. This subsystem will be 

the focus of the following discussion.

3.1 General Requirements Layout Description: Calorimeter Assemblies

The conceptual layout for the calorimeter coolant system is illustrated in figure 4. The coolant 

system is divided into two sections—the calorimeter and the supply. The calorimeter section is composed 

of all flow elements between the inlet and outlet manifolds on the test chamber side of the loop: the manifolds, calorimeter, copper tubing, stainless steel tubing, control valves, thermocouples, flow meters, thermal switches, and flow switches. Water for each calorimeter is routed between a common inlet manifold 

and outlet manifold adequately sized to eliminate pressure or flow restrictions. The specific calorimeter 

mass flow rate, selected to meet test requirements, is regulated by a hand valve located at the calorimeter 

exit; temperature and flow rate measurements are also taken at this location so that calorimeter power can 

be determined.

By definition, the supply contains everything that is not a part of the calorimeter and includes the 

recirculation components and connections to the facility coolant/electrical supply. This section maintains 

the required bulk water flow rate, temperature, filtration, and purity requirements. The current baseline 

is to make use of a single supply loop and set of manifolds to service all 16 calorimeters; however, this 

may be subdivided, depending on the final configuration of the environmental test chambers. Purified 

and/or treated water is loaded and maintained by the supply section using appropriate filters, deionizers, 
9

Flow Meter

Thermocouple

Pressure Gauge/Sensor

Relief Valve

Hand Valve

Flow Switch

Temperature Switch

Bottle Regulator

Evacuation

Interface

Argon Supply

K-Bottle

0–200 psi

0–200 psi

0–4000 psi

Set 100 psia

Set 100 psia

Water Pump

PG

PG

TC TC

TC

TC

4655 Int

Return

4655 Int

Return

Facility Water System

Calorimeter Section Supply Section FS

FS

FS

Temperature

Control Valve

Water

Treatment

Loop

Filter

Bypass Valve

Bypass Valve

FM

FM

32

Water

Reservoir

and

Expansion

Tank

Set 100 psia

L

P

Outlet

Manifold

Inlet

Manifold

Single Calorimeter Loop 

Illustrated for Clarity (x16)

Heat Pipe Outlet 

Flow Instrumentation

Heat Pipe Flow

Control Valves

Test Chamber

Calorimeter

Inlet Isolation Valve

*Numbers Denote Element Locations

 Used in Flow Analysis

1

3 2 4

5

6

7 8

9

10 11

12

15 14 13

16 19 18 17

20

21 22

23

24

25 26

27

28

29

30

31

Figure 4. General calorimeter water-cooling system layout (one calorimeter illustrated).

etc. to provide stable, long-term operation. To conservatively satisfy estimated cooling requirements, the 

selected supply section will be required to operate at an absolute pressure of ≈65 psia and to provide 

pure water at a minimum of 20 gpm at a pressure rise of 45 psi with a temperature of 300 K. Additional capacity will be built into the supply system to account for uncertainties. A complete flow analysis 

of the supply system will be used to determine final hardware requirements necessary for the procurement of a commercial turnkey coolant recirculation unit. Both the calorimeter and supply sections have 

initially been laid out with flow and temperature switches to interrupt the RF power supplies in the event 

of coolant circulation failure or an over-temperature condition. However, these switches are redundant 

to the flow and temperature measurements already located in the lines and may be dropped from the 

system, depending on the final data/control system used to monitor operations.

3.2 Coolant Water Quality

To maintain long-term reliable operation of the water-cooling system it is mandatory that issues 

such as scaling and corrosion be kept in check, leading to consideration of deionized (DI) water in the 

coolant loop. As the coolant system design matured, further investigation indicated that DI water creates problems when used with copper, the selected material for use in the calorimeter assemblies. Dortwegt and Pellegrino provide a good discussion of the potential issues and recommended practices.5,6

Suggested system values for water include conductivity <0.2 μS/cm, dissolved oxygen (DO)<20 ppb, and 

a pH range of 6.8 to 7.2. The DO concentration appears to be a major concern throughout. However, with 

the low pH range—driven by the resistivity requirement—copper corrosion is an unavoidable drawback. 

Several recommended practices that are pertinent to the current design include the following:
10

• Avoid the use of small (<1/8-in diameter) orifices for regulation of water flow rate.

• Employ ≈0.5-μm, point-of-use filtration in the primary loop; routine filter changes should be avoided 

to minimize the ingress of oxygen.

• Employ a branch treatment loop, if possible, with 0.05- to 0.1-μm filtration (≈5% of total throughput).

• Introduce all makeup water at a vacuum deaerator to remove all but trace DO.

• Maintain branch lines that are not used regularly with a continuous minimum flow to avoid DO 

accumulation.

• Flush pump seals to avoid direct impingement of copper particles on seal faces.

• Avoid use of flow-measuring equipment with moving parts, if economically possible.

• It is imperative that the source water for initial system filling and for makeup be free of bacterial 

contamination. Once filled, continuously circulate the system water; an additional ultraviolet (UV) treatment may be beneficial.

A significant driver for the systems described by Dortwegt and Pellegrino that led to the recommended practices above was the lower limit of resistivity, which in effect restricts the upper limit on pH.5,6

The water-cooling system application within this TP is not as sensitive to resistivity, so operation with 

water at a higher pH is a viable approach to alleviating concerns over copper corrosion. With this in mind, 

the above recommendations may be viewed as practices to follow if economically feasible, but not as strict 

requirements to be placed on the current calorimeter coolant system design.

Table 2 provides the ASTM standard specification for reagent water (D1193-99ε1).7 While these 

grades may be too stringent for the current application and too costly to maintain, type IV water with a pH 

at the upper bound (≈8) would be a good target for the initial fill and any makeup water. At a minimum, 

the supply section should contain a 0.5-μm filter, a deionizing cartridge, and a UV purification unit. If it is 

found that tighter control of water pH and DO content is required, then a purification loop may be added to 

the supply section, where a percentage (≈5 percent) of the total flow would be circulated and continuously 

purified.

Another coolant option that was investigated would employ a 30% to 50% mixture, by volume, 

of DOWTHERM 4000 and water.8,9 This approach is advantageous in that the fluid contains corrosion inhibitors that offer increased system protection without the needed expense for ancillary purifiers. 

DOWTHERM 4000 is an inhibited ethylene glycol-based heat transfer fluid manufactured by the Dow 

Chemical Company. Dow recommends using good quality water for dilution, containing only minute 

traces of calcium (<50 ppm), magnesium (<50 ppm), chloride (<25 ppm), and sulfate (<25 ppm), and 

<100 ppm of total hardness as CaCO3 (type III water). Dow also provides prediluted solutions, AMBITROL, if good quality water is not available. To date, the calorimeter and water system design have been 
11

Table 2. Water grades from the ASTM standard specification for reagent water.7

Type I Type II Type III Type IV

Electrical conductivity, maximum mS/cm at 298 K (25 °C) 0.056 1 0.25 5

Electrical resistivity, minimum MW•cm at 298 K (25 °C) 18 1 4 0.2

pH at 298 K (25 °C) * * * 5 to 8

Total organic carbon, maximum (mg/L) 50 50 200 No limit

Sodium, maximum (mg/L) 1 5 10 50

Chlorides, maximum (mg/L) 1 5 10 50

Total silica, maximum (mg/L) 3 3 500 No limit

Microbiological contamination—When bacterial levels need to be controlled, reagent grade types should be further classified as follows:

Type A Type B Type C

Maximum heterotrophic bacterial count 10/1,000 mL 10/100 mL 100/10 mL

Endotoxin, EU/mL** <0.03 0.25 NA

 *The measurement of pH in type I, II, and III reagent waters has been eliminated from the specification because these grades of water do not contain 

 constituents in sufficient quantity to significantly alter the pH.

**EU=Endotoxin units.

sized assuming the use of pure water as the heat transfer fluid. The addition of DOWTHERM 4000 to 

the water alters the fluid properties, the effects of which are presented in table 3. The computed flow 

rates and pressure drops are based on the listed constant fluid properties applied to heat pipe test case 

F(-4) (the 5,000-W test condition).

Table 3. Comparisons for water and solutions of water and DOWTHERM 4000 (concentrations given 

in percent volume) for F(-4) (all properties assumed constant).

Parameter Water 30% DOWTHERM 4000 50% DOWTHERM 4000

ρ (kg/m3) 992 1,044 1,075

cp (kJ/kg-K) 4.2 3.7 3.3

k (W/m-K) 0.63 0.47 0.39

μ (Pa-s) 0.00065 0.0013 0.0023

Re 2,329 2,678 2,816

Q (gpm) 0.78 1.7 3.1

∆P (psi) 4.2 19.6 64.3

The flow rates and pressure drops resulting from the addition of DOWTHERM 4000 represent 

a substantial increase over that of pure water and are unacceptable. Vendor discussions indicate that it is 

also possible to tailor a fluid, e.g., the antifreeze protection afforded by DOWTHERM 4000 is not needed, 

consisting of water and corrosion inhibiters that provide heat transfer properties nearer that of pure water. 

This possibility will be investigated further and system design/performance impacts assessed; however, 

water will remain the baseline coolant. The final coolant selection will be based on recommendations from 

the commercial vendor selected (through open procurement) to supply the heat removal water recirculation systems.
12

3.3 Sizing and Performance

Spreadsheet-level analyses have assessed the performance of the water-cooling system. Simplifying assumptions were necessary due to the preliminary status of the design. However, the intent was 

to provide a conservative estimate with sufficient fidelity to facilitate component sizing. As the design 

matures, so will the spreadsheet analyses. In addition, a more detailed model could be developed with 

the Generalized Fluid System Simulation Program (GFSSP) to accurately predict system performance.10

More detailed multidimensional, e.g., finite element, etc., analyses will be performed as required to assess 

any design issues or to guide optimization of component-level performance.

The properties for water are generated with software that incorporates the IAPWS-95 formulation for the thermodynamic properties of pure water into an ActiveX component in Visual Basic 

6.0.11 The ActiveX component is called by Microsoft Excel, which is used to create the current cooling 

system model.

Table 4 provides a general description of each flow element modeled. It includes both the calorimeter section (elements 6 –20) and those components of the supply section (elements 1–5 and 21–32) 

that are a part of the closed recirculation cooling system. While purchasing of a commercial turnkey 

system for the supply section is the preferred option, modeling of the supply section assists in determining 

specifications for the turnkey system and provides an initial baseline design should it become an MSFCbuilt supply section.

3.3.1 Calorimeter Section

Table 5 provides a listing of the primary model parameter values and the predicted pressure drops. 

The flow elements are consistent with figure 4 and are in order beginning at the pump exit. The pressure 

drops are computed using either of the following equations:

ΔP = f L

dh

⎛

⎝

⎜

⎞

⎠

⎟

ρv 2

2

Γn (1)

or

ΔP = K ρv 2

2

Γn , (2)

where the hydraulic diameter, dh, is given by the tube diameter for a circular tube; for an annular gap, 

dh=do –di. The viscosity ratio, Γ, is used to account for temperature-induced variations in viscosity 

whenever heat is transferred to or from the fluid. The value of n used in all results is that recommended 

by Furukawa, n=–0.25.12 (Note that other references suggest different values.) All calculations assume 

that the thin flow annulus, i.e., flow element 11, is the only element that transfers heat, with all other 

elements being adiabatic, i.e., Tw=Tin=Tout. Friction factors are computed using formulas given by Furukawa.12 For a circular tube or pipe,
13

Table 4. Flow element descriptions for the water-cooling system spreadsheet model.

Flow Element Flow Element Description

1 Pipe from pump exit to inlet manifold

Supply Section

2 Ball valve

3 Turbine flow meter

4 Flow switch

5 Pipe exit into inlet manifold

6 Tube entrance from inlet manifold to calorimeter

Calorimeter Section

7 Tube from inlet manifold to calorimeter

8 Ball valve

9 Tube exit into calorimeter

10 Calorimeter entrance into thin annulus

11 Calorimeter thin annulus

12 Calorimeter turnaround

13 Calorimeter thick annulus

14 Calorimeter exit out of thick annulus

15 Tube entrance from calorimeter to outlet manifold

16 Tube from calorimeter to exit manifold

17 Ball valve

18 Turbine flow meter

19 Flow switch

20 Tube exit into outlet manifold

21 Pipe entrance from outlet manifold to heat exchanger

Supply Section

22 Pipe from outlet manifold to heat exchanger

23 Pipe exit into heat exchanger

24 Ball valve

25 Heat exchanger

26 Pipe entrance from heat exchanger to expansion tank

27 Pipe from heat exchanger to expansion tank

28 Ball valve

29 Pipe exit into expansion tank

30 Pipe entrance from expansion tank to pump inlet

31 Pipe from expansion tank to pump inlet

32 Filter

f = 64

Re

if Re ≤ 2,000 , (3)

f = 0.2088 − 0.1868 Re

1,000

⎛

⎝

⎜ ⎞

⎠

⎟ + 0.0624 Re

1,000

⎛

⎝

⎜ ⎞

⎠

⎟

2

− 0.00656 Re

1,000

⎛

⎝

⎜ ⎞

⎠

⎟

3

 if 2,000<Re<4,000 , (4)
14

Table 5. Key parameters for the calorimeter section spreadsheet model for test case F(-4).

Flow Element

Inputs Outputs

Type Tw (K) Tin (K) Tout (K) d or di (m) do (m) L (m) DH (m) Re f K DP (psi)

6 0 300 300 300 0.00775 0 9,321 0.0322 0.5 0.038

7 0 300 300 300 0.00775 1.219 0.762 9,321 0.0322 5.1 1.469

8 0 300 300 300 0.00775 0.023 0 9,321 0.0322 0.1 0.007

9 0 300 300 300 0.00775 0 9,321 0.0322 1 0.077

10 1 300 300 300 0.02019 0.02096 0 1,755 0.0547 0.5 0.141

11 1 396 300 324.4 0.02019 0.02096 0.330 0 2,254 0.0336 14.6 4.109

12 1 324.4 324.4 324.4 0.02019 0.02096 0.038 0 2,797 0.0416 2.1 0.591

13 1 324.4 324.4 324.4 0.02396 0.03026 0.330 0 2,123 0.0452 2.4 0.006

14 1 324.4 324.4 324.4 0.02396 0.03026 0 2,123 0.0452 1 0.002

15 0 324.4 324.4 324.4 0.00775 0 14,857 0.0287 0.5 0.039

16 0 324.4 324.4 324.4 0.00775 1.829 –0.762 14,857 0.0287 6.8 –0.546

17 0 324.4 324.4 324.4 0.00775 0.023 0 14,857 0.0287 0.1 0.007

18 0 324.4 324.4 324.4 0.00775 1.549 0 14,857 0.0287 5.7 0.444

19 0 324.4 324.4 324.4 0.00775 0.054 0 14,857 0.0287 0.2 0.016

20 0 324.4 324.4 324.4 0.00775 0 14,857 0.0287 1 0.077

6.48

and

f=0.3164 Re–0.25 if Re≥4,000 , (5)

while for an annular gap,

f = 64

Reψ if Re < Re* , (6)

and

f=0.304e0.1Re–0.25 if Re≥Re* , (7)

where

ε = di

do

, (8)

ψ = (1− ε)

2

1+ ε

2 +

1− ε

2 ( )

ln(ε)

⎡

⎣

⎢

⎢

⎤

⎦

⎥

⎥

, (9)
15

and

Re*=1,250y1.33 e–0.133 . (10)

To account for secondary losses, e.g., entrance effects, exit effects, etc., and flow elements that 

are not tubes or annular gaps, e.g., valves, flow meters, etc., the Crane Report is used to approximate 

the appropriate resistance coefficients, K=f (L/dh).13 Table 5 lists K for all flow elements in the cooling 

system design. 

For the tube and annular gap elements, K is computed using equations (3) through (10). All 

entrance effects are approximated with K=0.5, i.e., treated as a sharp-edged pipe entrance, while K=1, 

i.e., treated as a pipe exit, is used to account for exit effects. All valves are treated as ball valves where 

K=3 f

T and the friction factor in the fully turbulent zone, fT, is calculated with the previous relations 

for a circular tube at the appropriate flow rate and diameter. Defining K as a function of f

T effectively 

defines an effective (L/dh), which is how these values are input into the spreadsheet model. For example, while (L/dh) is calculated for the tube and annular gaps, for the ball valves, the value of L is input 

as 3 dh. In a similar manner, the constant K values defining entrance and exit effects are input into the 

spreadsheet by replacing the computed K with the appropriate constant values. The resistance coefficients for the transition from the thin annular gap of the calorimeter to the thicker annular gap is treated 

as a closed pattern return bend (K=50 f

T). The resistance coefficients for the flow meters (K=200 fT), 

the flow switches (K=7 f

T), the heat exchanger (K=500 fT), and the filter (K=200 fT) are all rough estimates at this point and will need refinement as the design matures. In addition, the calculation assumes 

that there is no significant pressure drop in the inlet and outlet manifolds. The manifold diameters will 

be relatively large to minimize any effects they might have on flow distribution between the calorimeters. As a result, there will be very little dynamic pressure and very little change in static pressure. 

The current baseline is a 4-in-diameter manifold. There are 16 units, i.e., heat pipes, to be tested, resulting in unique operating conditions through each of the 16 calorimeters. Table 5 represents the conditions 

corresponding to unit F(-4), which yields the largest pressure drop. Table 6 gives the pressure drops 

through each of the 16 defined test conditions. Note that there are seven G-series heat pipes to be tested, 

all of which yield the same calculated pressure drop.

The maximum pressure drop occurs through the F(-4) calorimeter; this value will be used to size 

the remaining water subsystem. The appropriate flow splits will be obtained through the other calorimeters 

by adjustment of the individual flow control valves.

3.3.2 Supply Section

Table 7 provides a listing of the primary model parameter values along with the predicted pressure drops for both the calorimeter and supply sections of the recirculating coolant loop. The pressure 

drops are computed at the operational conditions for test case F(-4) since these represent the largest 

pressure drop. The flow element pressure drop calculations utilize equations (1) through (10).

Figure 5 illustrates the system pressure-flow curves for two cases with primary feed system piping 

diameters of 1 and 2 in (with wall thickness of 0.083 in). The calorimeter inlet and outlet manifolds are set 

at 4-in diameters to eliminate potential flow restrictions. The tubing for each calorimeter, i.e., from inlet
16

Table 6. Calculated pressure drops for each of the 16 heat pipe test cases.

Units DP (psi)

G series 5.11

F(-4) 6.48

F(-3) 0.74

F(-2) 4.67

F(-1) 5.66

F(1) 4.89

F(2) 5.50

F(3) 4.44

F(4) 5.06

F(0) 5.11

manifold to outlet manifold, has a diameter of 0.375 in, with a wall thickness of 0.035 in. In the actual 

installation, the supply section piping may very well have some combination of diameters—and reduction 

at fittings and components—but in bounding the problem, the assumption is made that the diameter will 

reside between 1 and 2 in.

In summary, the water-cooling system has been characterized through spreadsheet-level analyses, 

making use of simplifications and assumptions that are believed to be suitable for the preliminary status 

of the design. The intent was to establish a simple model incorporating representative flow components 

with a conservative treatment and additional margin added to account for calculation uncertainty and the 

addition of unanticipated ancillary cooling requirements. The results of this assessment shall be used to 

set procurement specifications for the supply side recirculation system. The final pump selection should 

be capable of delivering a minimum of 20 gpm at a supply pressure of 45 psi.

3.3.3 Calorimeter Exit Flow Temperature Measurement

To minimize the risk of water leakage inside the environmental test chambers, all temperature 

probes in contact with water, using mechanical connections, will be located outside of the chamber. 

This introduces an unknown regarding measurement of the calorimeter outlet water temperature, since 

it would be positioned outside the test chamber (≈18 in downstream of the calorimeter). To analyze 

possible heat loss from the calorimeter tubes, an assessment is made using convection and radiation 

heat transfer relations and properties obtained from Incropera.14 It is assumed that the primary source 

of heat transfer will be between the 300 K supply tubes/chamber surfaces and the warmer return tubes. 

The heat loss from the return tubes between the calorimeter exit and the measurement location external to the test chamber is assessed using appropriate parameters. The distance between the exit and measurement locations is assumed to be ≈18 in, the emissivity of copper is 0.45, the outside diameter of the 

copper tube is 0.375 in, and all other surfaces are assumed to be at a temperature of 300 K. Test setup F(-3) 

has the maximum outlet temperature condition of 335 K, which will result in the greatest heat loss. Under 

these conditions, 1.75 W of heat is radiated from the tube surface between the calorimeter exit and external 

measurement location. For the natural convection contribution to heat loss, due to a He environment, the 

Nusselt, Rayleigh, and Grashof numbers are given by equations (11) through (13):
17

Table 7. Key parameters for the calorimeter and supply section spreadsheet model.

Flow Element

Inputs Outputs

Type Tw (K) Tin (K) Tout (K) d or di (m) do (m) L (m) DH (m) Re f K DP (psi)

1 0 300 300 300 0.02118 1.829 0 40,862 0.0223 1.92 0.379

2 0 300 300 300 0.02118 0.064 0 40,862 0.0223 0.07 0.013

3 0 300 300 300 0.02118 4.237 0 40,862 0.0223 4.45 0.878

4 0 300 300 300 0.02118 0.148 0 40,862 0.0223 0.16 0.031

5 0 300 300 300 0.02118 0 40,862 0.0223 1 0.197

6 0 300 300 300 0.00775 0 9,321 0.0322 0.5 0.038

7 0 300 300 300 0.00775 1.219 0.762 9,321 0.0322 5.07 1.469

8 0 300 300 300 0.00775 0.023 0 9,321 0.0322 0.1 0.007

9 0 300 300 300 0.00775 0 9,321 0.0322 1 0.077

10 1 300 300 300 0.02019 0.02096 0 1,755 0.0547 0.5 0.141

11 1 396 300 324.4 0.02019 0.02096 0.330 0 2,254 0.0336 14.55 4.109

12 1 324.4 324.4 324.4 0.02019 0.02096 0.038 0 2,797 0.0416 2.08 0.591

13 1 324.4 324.4 324.4 0.02396 0.03026 0.330 0 2,123 0.0452 2.37 0.006

14 1 324.4 324.4 324.4 0.02396 0.03026 0 2,123 0.0452 1 0.002

15 0 324.4 324.4 324.4 0.00775 0 14,857 0.0287 0.5 0.039

16 0 324.4 324.4 324.4 0.00775 1.829 –0.762 14,857 0.0287 6.77 –0.546

17 0 324.4 324.4 324.4 0.00775 0.023 0 14,857 0.0287 0.09 0.007

18 0 324.4 324.4 324.4 0.00775 1.549 0 14,857 0.0287 5.73 0.444

19 0 324.4 324.4 324.4 0.00775 0.054 0 14,857 0.0287 0.2 0.016

20 0 319.8 319.8 319.8 0.00775 0 60,243 0.0202 1 0.199

21 0 319.8 319.8 319.8 0.02118 0 60,243 0.0202 0.5 0.099

22 0 319.8 319.8 319.8 0.02118 1.829 0 60,243 0.0202 1.74 0.347

23 0 319.8 319.8 319.8 0.02118 0 60,243 0.0202 1 0.199

24 0 319.8 319.8 319.8 0.02118 0.064 0 60,243 0.0202 0.06 0.012

25 0 319.8 319.8 319.8 0.02118 10.592 0 60,243 0.0202 10.1 2.007

26 0 319.8 319.8 319.8 0.02118 0 60,243 0.0202 0.5 0.099

27 0 319.8 319.8 319.8 0.02118 1.219 0 60,243 0.0202 1.16 0.231

28 0 319.8 319.8 319.8 0.02118 0.064 0 60,243 0.0202 0.06 0.012

29 0 319.8 319.8 319.8 0.02118 0 60,243 0.0202 1 0.199

30 0 319.8 319.8 319.8 0.02118 0 60,243 0.0202 0.5 0.099

31 0 319.8 319.8 319.8 0.02118 1.219 0 60,243 0.0202 1.16 0.231

32 0 319.8 319.8 319.8 0.02118 4.237 0 60,243 0.0202 4.04 0.803

10.937

NuL = h L

k = 0.68 +

0.67 RaL

1/4

1+ (0.492/Pr)

9/16 ⎡

⎣ ⎤⎦

4/9 , (11)

RaL=GrLPr , (12)
18

Volume Flow Rate (gpm)

0 5 10 15 20

40

30

20

10

0

D = 1 in

D = 2 in

Pressure Drop (psi)

Figure 5. Computed system pressure-flow curves for water-cooling system.

and

Gr

L = gβ Ts −T ( ∞)L3

v

2 , (13)

where

L = 0.4572 m

g = 9.807 m/s2

k = 0.165 W/m-K

v = 0.000149 m2/s

Pr = 0.68

b = 0.003252 K–1

Ts = 335 K

T∞ = 300 K.

The resulting Grashof number is GrL=4.65×106, the Rayleigh number is RaL=3.16×106, and 

the Nusselt number is NuL=22, resulting in a convection coefficient of h=8 W/m2K. The heat loss due 

to natural convection is ≈3.85 W; the resulting total heat loss—given by the sum of radiation and convection—is 5.6 W. When this heat loss is compared to the total extracted power of 1,000 W for test case F(-3), 

it represents <0.6% of the power with a corresponding temperature drop in the flowing water of 0.2 K. 

These effects are negligible and can be further reduced by the addition of multifoil insulation covering 
19

the return tubing inside and outside the test chamber. In addition, thermocouples will be mounted on the 

surface of the calorimeter return tubing ≈2 cm from the calorimeter exit. The internal and external temperature data can be compared to determine overall accuracy of the measurement technique in addition to 

serving as a backup.

3.4 General Fill and Test Procedures

A set of general procedures pertaining to the initial fill and conditioning for the heat pipe calorimeter water-cooling subsystem is outlined. Before loading, all system plumbing should be cleaned thoroughly and flushed to remove all particulates. A trickle purge using dry nitrogen can be placed on the 

system prior to filling to sweep any remaining volatile materials and to maintain the system at an acceptable condition if not in use for extended periods of time. The purge should be shut down and isolated prior 

to loading the system with water coolant. This outline will serve as a general approach and will be adjusted 

to match specific operational parameters required by the selected turnkey water coolant recirculation unit.

3.4.1 Flushing Primary Flow Circuit

The following procedure is used for flushing the primary flow circuit:

(1) Prior to flushing the primary flow circuit, all individual flow circuit tubing and pipe components should have been flushed (during assembly) to remove the bulk of potential debris.

(2) Close all heat pipe calorimeter inlet and outlet isolation valves.

(3) Open all primary water flow system isolation valves.

(4) Open expansion tank pressure relief valve and fill the system with water—of appropriate 

purity—through the expansion tank.

(5) Turn on pump and cycle water through the complete circuit using the inlet to outlet manifolds 

bypass line. This will purge trapped air and moves remaining debris to the system filter.

(6) Examine all fittings, welds, and instrumentation for water leaks. Tighten or replace as 

necessary.

(7) Check water level in expansion tank and add water if necessary.

(8) Shut down pump after operating for 8 hr.

(9) Drain system through inlet/outlet manifold and expansion reservoir drains.

(10) Remove and inspect expansion tank strainer and inline filters. Clean and/or replace if 

necessary.

(11) If significant debris is found, repeat this procedure until all inline filters and strainers 

are clean.
20

3.4.2 Flushing Complete Flow Circuit

The following procedure is used for flushing the complete flow circuit:

(1) Prior to flushing the complete flow circuit, all primary flow circuit tubing and pipe should 

have been flushed and verified to be clean per inspection of inline filters and strainers.

 (2) Open all heat pipe calorimeter inlet and outlet isolation valves.

 (3) Open all primary water flow system isolation valves.

 (4) Close inlet to outlet manifold bypass isolation valve.

 (5) Open expansion tank pressure relief valve and fill the system with water—of appropriate 

purity—through the expansion tank.

 (6) Turn on pump and cycle water through the entire flow circuit, including the heat pipe calorimeters. This will purge trapped air and moves remaining debris to the system filter.

 (7) Adjust pump bypass valve to achieve flow in calorimeters greater than that required for 

testing.

 (8) Examine all fittings, welds, and instrumentation for water leaks. Tighten or replace as 

necessary.

 (9) Check water level in expansion tank and add water if necessary.

(10) Monitor flow rate through each of the calorimeters and verify that full coolant flow is maintained. Flow drop will indicate onset of potential plugging.

(11) Shut down pump after operating for 4 hr.

(12) Drain system through inlet/outlet manifold and expansion reservoir drains.

(13) Remove and inspect expansion tank strainer and inline filters. Clean and/or replace if 

necessary.

(14) If debris is found, repeat this procedure until filters and strainers are clean.

3.4.3 Fill Water-Cooling Circuit

The following procedure is used to fill the water-cooling circuit:

 (1) Open all system isolation, bypass, and flow control valves.
21

 (2) Open expansion tank pressure relief valve and fill system with water—of appropriate purity—

through the expansion tank.

 (3) Turn on pump and circulate water to purge all trapped air.

 (4) Monitor flow through the primary loop and all calorimeters to verify that it meets or exceeds 

the nominal design flow requirements. Adjust pump bypass valve if needed to achieve desired conditions.

 (5) Examine all fittings, welds, and instrumentation for water leaks. Tighten or replace as necessary to repair.

 (6) Operate pump system for 2 to 4 hr.

 (7) As trapped air is purged, audible indications of air bubbles moving through the system will 

cease. Close the inlet to outlet manifold bypass isolation valve.

 (8) Check water level in expansion tank and add water if necessary.

 (9) Shut down pump after operating for 4 hr.

(10) Close expansion tank fill cap and relief valve.

(11) Close purge valve and pressurize the tank to 65 psia with Ar.

(12) Turn on pump and operate for 2 to 4 hr.

(13) Examine all fittings, welds, and instrumentation for water leaks. Tighten or replace as 

necessary.

(14) Shut pump down after operating for 4 hr. Close pump and expansion tank isolation valves.

3.4.4 Deaerate the Cooling Water Circuit

The following procedure is used for dearating the cooling water circuit:

(1) Prior to this operation, the water circuit should be filled and all trapped air purged. The 

expansion tank should be equipped with a heater, either an external blanket or internal element, and 

insulation.

(2) Open pump and expansion tank isolation valves. Open bypass valves. Verify that all other 

circuit flow valves are open.

 (3) Turn on pump and monitor water flow rate and pressure in both the primary and calorimeter 

sections.
22

 (4) Turn on expansion tank heater and bring water to a target temperature of approximately 

350 to 400 K, i.e., below the boiling temperature of 414.8 K at 55 psi. 

 (5) Open vacuum isolation to provide a reduced pressure in the 10 to 14 psia range on the expansion tank ullage. This removes any dissolved gases that are released during heating. Ullage pressure and 

heating may need to be adjusted to prevent pump cavitation.

 (6) Continue heating and circulation for up to 2 hr.

 (7) Isolate the vacuum system and turn off expansion tank heater; allow system to cool.

 (8) Maintain a blanket circuit pressure by adding Ar as necessary—up to a level of 65 psia.

 (9) Check water level in expansion tank and refill as necessary by depressurizing tank and filling 

through the tank fill cap. Minimize exposure to the atmosphere and maintain a slight positive Ar purge 

pressure inside the expansion tank, if possible, during the operation. Repressurize tank to 60 psi with Ar.

(10) If the required fill amount is >20% of the expansion tank volume, repeat the deaerating 

process.

3.4.5 Normal Water Circuit Operation

The following procedure is used for the normal water circuit operation:

 (1) Verify that water circuit pressure is at 65 psia.

 (2) Open expansion tank and pump isolation valves.

 (3) Verify that flow circuit valves on the calorimeter section are open.

 (4) Turn on water pump and check for leaks.

 (5) Monitor all circuit flow rates, pressures, and temperatures.

 (6) Adjust pump flow control valve—and bypass valve, if necessary—to achieve a flow that 

is between 100% and 120% of the calorimeter full-flow requirement.

 (7) Adjust each calorimeter flow control valve—located at the calorimeter exit—in order from 

the highest to lowest flow split. Iterate until flow in each of the calorimeters is within 10% of the target 

conditions specified in the test matrix.

 (8) Monitor total system flow and increase—by closing the pump bypass valve—should the total 

flow be insufficient to maintain full flow conditions on all calorimeters.

 (9) Continually monitor all water circuit instrumentation.
23

(10) Periodically verify water level in expansion tank.

(11) Water system may require periodic adjustment once RF supplies are engaged and power 

is extracted by the calorimetry system.

(12) Verify that the control system is engaged to shut down RF supplies should there be a loss of 

coolant flow or an over-temperature condition on any of the calorimeter branches or primary flow circuit.

3.5 General Procurement Specifications

Primary components for the calorimeter water-cooling subsystem have been identified. (Instrumentation is included in sec. 6.) The procurement of these items will be issued by MSFC using full 

and open competition. The final evaluation of vendor quotes will be based on the lowest cost technically 

acceptable approach. Appendix E provides a partial listing of components as identified at the current time. 

Categories include the following:

(1) A closed-loop recirculation system with sufficient cooling capacity (≈60 kW) to service 

all heat pipe calorimeters. This unit will be a turnkey commercial off-the-shelf item such as the Sentry 

Equipment Compact Cooling Water Isolation Skid CWIS-35 with a reservoir and flow sensors designed 

for continuous operation. Typical vendor cost is $12K to $15K, available within 6 to 8 wk.

(2) Inline water treatment components, including a UV element, DI cartridge, filters, and strainers. These are commercial off-the-shelf items, typically available within 4 to 6 wk with an estimated cost 

range of $2K to $4K, depending on the final number of components and desired operational life. 

(3) Valves, including hand and remote operated, for isolation and control of water circuit components. These will be a combination of ball-and-plug-type valves making use of either compression 

or pipe fittings. In addition, pressure relief valves are required. Valve material will be stainless steel with 

soft goods compatible with water service. Typical vendor delivery is 2 to 4 wk with an estimated cost 

range of $15K to $20K.

(4) A number of plumbing components, such as tubing, fittings, vacuum valves, dial pressure 

gauges, and structural materials, are required. All of these are commercial off-the-shelf items, available 

from a variety of vendors and typically available within 4 wk or less. The sum of these components 

is expected to range from $7K to $10K.

(5) Part fabrication and assembly at machine shops as required to complete system. On-demand 

basis with estimated cost in the $5K to $8K range.
24

4. RADIO FREQUENCY HEATING SYSTEM

Radio frequency heating has been selected as the most favorable technique for powering the life 

test heat pipes. Several key benefits associated with using RF heating include the following:

• Provides a noncontact method of imposing evaporator power.

• Provides uniform power input over a small, well-defined surface area.

• Provides sufficient standoff between the RF inductor coils and the heat pipe to allow for easy heat pipe 

installation/removal.

• Provides superior durability suitable for long-duration operation.

The complete RF heater configuration selected for this project includes a number of component 

subsystems: the RF inductive coil assemblies, the interconnecting busswork between the coil elements 

and the power supplies, the RF power supplies, coolant flow to power supplies/inductive coil assemblies, 

and instrumentation. The following sections detail the approach, conceptual design, and hardware layout 

necessary for implementation.

4.1 General Heat Pipe Test Arrangement

The current baseline test matrix (table 8) identifies performance conditions for 16 heat pipes. To 

make better use of test hardware and to facilitate integration and test operations, these heat pipes can 

Table 8. Proposed heat pipe life test matrix.

Test 

Identifier

Power Level 

(W)

Heat Pipe 

Temperature 

(K)

Gas Gap

Mixture

Gas Gap 

Width 

(in)

Coolant 

Flow 

(gpm)

Coolant Delta 

Temperature 

(K)

Number of 

Units

G-Series 3,000 1,273 He-32%Ar 0.025 0.61 18.8 7

F(-4) 5,000 1,273 He-6%Ar 0.025 0.77 24.6 1

F(-3) 1,000 1,273 He-32%Ar 0.103 0.11 35.2 1

F(-2) 3,000 1,373 He-32%Ar 0.031 0.56 21.1 1

F(-1) 3,000 1,173 He-32%Ar 0.021 0.65 17.6 1

F(0) 3,000 1,273 He-32%Ar 0.025 0.61 18.8 1

F(1) 2,000 1,223 He-32%Ar 0.037 0.54 13.9 1

F(2) 4,000 1,223 He-32%Ar 0.017 0.67 22.8 1

F(3) 2,000 1,323 He-32%Ar 0.046 0.50 15.2 1

F(4) 4,000 1,323 He-32%Ar 0.020 0.63 24.0 1
25

be arranged into groupings with similar or scalable power and environmental requirements. Initial investigations indicate that one possible configuration involves dividing the 16 heat pipes across four separate 

test clusters; each of these clusters is fed by its own RF power supply. Three of these clusters make use 

of a configuration composed of five inductive coils laid out in a pentagonal arrangement (coil centerlines 

set at 72-deg intervals). Figure 6 illustrates this configuration with coil positions set to maximize the distance between each unit—limiting inductive coupling—while providing sufficient workspace to configure 

the hardware within the test chamber. The coils are supported by a structural element attached to the test 

chamber. The RF busswork passes through an insulated test chamber penetration/feedthrough. 

Test Chamber Shell

RF Inductive Coils (×5)

Cluster Busswork and

Structure Support

RF Feedthrough

Position

1

Position

3

Position

2

Position

4

Position

5

Figure 6. General test chamber layout showing the typical five-position RF coil cluster.

The coils within each cluster will be connected in series, with the coil geometry adjusted such that 

two conditions are possible—uniform power across all coil elements and fixed power variation between 

various coils within the cluster. Two of these clusters—representing 10 heat pipes—are to be sized for 

a uniform power condition set to an average power of 3 kW per heat pipe (neglecting environmental 

losses). This covers the complete G-series (seven units) and F-series heat pipes F(-2), F(-1), and F(0). 

The third cluster, composed of the remaining F-series heat pipes with the exception of the F(-4) heat pipe, 

provides a variation in power with inductive coil geometry adjusted to provide a scaling of 1, 2, and 4 kW 

in heat pipe power.

The F(-4) case will be contained in a separate test chamber to accommodate the higher 5-kW 

power level and to allow operation with a different atmospheric composition (higher He concentration). This chamber will also be equipped with its own inductive coil feed from a separate power supply. 

Table 9 provides a listing of each heat pipe’s position within the four test chamber RF clusters. 
26

Table 9. Position of heat pipes within test chamber clusters.

Test Chamber 

Cluster Position 1 Position 2 Position 3 Position 4 Position 5

1

2

3

4

G(1)

G(6)

F(-3)

F(-4)

G(2)

G(7)

F(1)

 –

G(3)

F(0)

F(3)

 –

G(4)

F(-1)

F(2)

 –

G(5)

F(-2)

F(4)

 –

The RF inductive coil assemblies will be designed to maintain an approximate constant power distribution across all positions, even if positions are left open; i.e., no heat pipes are placed within the coils. 

This is a necessary operating condition since G-series heat pipes will be extracted at regular intervals for 

destructive evaluation, resulting in fewer units in test chamber clusters 1 and 2 over time. It may be advantageous to rearrange the clusters to consolidate heat pipes, rebalancing the electrical load and reducing the 

amount of necessary support hardware.

4.2 General Inductive Coil Geometry

The general layout of a possible RF inductive coil configuration is illustrated in figure 7. The unit 

consists of a three-turn inductor covered with a flux concentrator to minimize coupling with the water 

calorimeter and adjacent RF coils in the cluster while providing a more uniform and symmetrically heated 

area on the heat pipe surface. The assembly is composed of three cylindrical copper elements with a 

continuous RF power feed tube brazed to their surfaces. Each cylindrical element has a longitudinal split 

and is positioned with a gap to its neighboring element to prevent shorting. The flux concentrator is also 

constructed of three sections—one for each element—and surrounds the coil arrangement. A nonconductive potting material—vacuum compatible—will be used to assemble the complete RF inductive coil 

assembly.

When arranged in a pentagonal cluster, as illustrated in figure 8, each coil will be set on a 9-cm, 

center-to-center spacing with respect to its adjacent coils. These coils will be connected in series with 

coolant water circulated through the RF feeds. Water coolant flow—supplied at the RF power supply—

should be adjusted to maintain the coil assembly temperature at <350 K to minimize the potential for 

damage. The three-turn inductive coils illustrated in figures 7 and 8 are sufficient to satisfy the uniform 

power requirements for test chamber clusters 1 and 2. The third test chamber cluster will require inductors with varied turns to satisfy power requirements. The initial baseline setup will require a two-turn 

inductor for the 1-kW position, a three-turn inductor for the 2-kW positions, and a four-turn inductor 

for the 4-kW positions.

The power to each cluster can be provided by a standard off-the-shelf insulated RF feedthrough 

such as that marketed by Insulator Seal Incorporated (part No. 9512010). The design of the structural 

support for the RF coil cluster arrangement will be performed in conjunction with the overall RF inductor system design and fabrication, to be issued as an open procurement to industry. However, at a minimum, the support must possess adequate rigidity to maintain positional tolerance, keeping the heat pipes 

centered in the coils, while providing sufficient strain relief on the feedthrough to prevent a possible 

failure (cracked insulator).
27

Coil Longitudinal

Split

Coil Support 

and RF Supply 

and Return

Flux Concentrators (Three Sections)

RF Inductive Coils (Three Turns)

Element Gap

Figure 7. Sample RF inductive coil layout.

Heat Pipe

Location

RF Feed/Return and

Coil Cluster Support

Coil Interconnecting

Busswork and Cooling

Figure 8. General test chamber section/RF inductive coil configuration.
28

4.3 Inductive Coil Sizing and Performance Parameters

A set of first-order assessments was made of the inductive coil layout to examine critical issues 

such as frequency and coil voltage requirements to achieve efficient power transfer to the heat pipe 

evaporator sections. These coil parameters are driven by geometric considerations, such as the heat 

pipe wall thickness, RF coil to heat pipe gap, and the number of inductor coil turns. Other parameters 

that must be considered include the potential for voltage breakdown, since testing will be performed in 

a low-pressure environment, and thermal losses to the inductive coil, which increases input power 

demands. The following sections detail the initial assessments performed to rough out the operational 

envelope for the RF power system.

4.3.1 Voltage Breakdown Considerations

A significant problem in low-pressure systems with imposed high voltage is a condition referred 

to as voltage breakdown. This condition can lead to a glow discharge with the potential for damaging 

the part under test, either by sputter or direct arc impingement. To minimize the potential for voltage 

breakdown, it is desirable to keep the voltage drop to ground across the series inductor string as low 

as possible. Direct current (DC) voltage breakdown potentials, referred to as Paschen curves, have been 

determined for various gases and are documented in the literature. Figure 9, generated from data collected 

by Weston, illustrates the variation of breakdown voltage for He, Ar, and He-Ar mixtures as a function of 

the product of pressure and gap distance.15 Weston reported the lowest measured DC breakdown potentials for the He-Ar gas mixtures of interest, improving the conservative nature of estimates made in this 

work.

Breakdown Potential (V)

Pressure x Distance (torr-cm)

Helium

Argon

30% Argon

5% Argon

1,000

100

1 10 100

Figure 9. Direct current breakdown voltages in pure He, Ar, and He-Ar mixtures (Paschen curves).
29

For the conditions outlined in baseline testing (He-32%Ar and He-6%Ar), the typical total test 

chamber pressure will be maintained at ≈76 torr. Breakdown voltages are estimated using the curves 

shown in figure 10 for various gap distances. For example, with a heat pipe to inductive coil gap width 

set to 0.64 cm (0.25 in), the resulting breakdown potential is ≈300 V for a 5%Ar mixture and ≈450 V 

for a 30%Ar mixture. To maintain a safe margin against the potential onset of voltage breakdown, 

a factor of 2 reduction in applied voltage will be imposed on the RF system. This results in a maximum 

inductive coil voltage drop of 225 V for the five-position clusters and 150 V on the single-coil setup to be 

used for test case F(-4). An additional consideration is the effect of the RF frequency on the breakdown 

voltage. Radio frequency power is typically used to accelerate the onset of glow discharge, lowering the 

breakdown potential. However, the frequency range to achieve an appreciable decrease in breakdown 

voltage for gases such as He and Ar is in the 5+ MHz range.16,17 The He/Ar mixture is expected to follow 

the same trends. Breakdown Potential (V)

1,000

900

800

700

600

500

400

300

200

100

0

Gap Width (cm)

0 0.2 0.4 0.6 0.8 1 1.2 1.4

He-30%Ar

He-5%Ar

Figure 10. Breakdown voltage range with a total operating pressure of 76 torr.

For operating frequencies in the 10- to 50-kHz range, such as the proposed RF system, there 

should be little reduction in the breakdown potentials. However, checkout tests will be performed 

on a characteristic geometry using an inductive coil assembly for verification. If it is found that the 

planned test pressure is close to a breakdown threshold, the chamber pressure will be increased. 

(A 50-torr increase provides a 100-V improvement for the 32%Ar condition.) The high test chamber 

pressure (above 10 torr) was selected to place operation to the right of the Paschen minimum (fig. 9). 

In the event of a chamber leak, an increase in pressure will always result in an increasing breakdown voltage requirement, eliminating the potential for arcing. 
30

4.3.2 Thermal Loss Considerations (Heat Pipe to Inductive Coil)

Although the RF inductive coil is a noncontact system, thermal losses from the heat pipe to 

the coil by both radiation and gas conduction are still present. It is important to characterize this heat 

transfer to the coil arrangement so that adequate cooling can be provided. To minimize this thermal loss 

it is advantageous to place the coil as far from the heat pipe as possible. This also carries the added benefit 

of further reducing the potential for voltage breakdown, increasing the product of pressure and distance. 

However, as the separation gap between the heat pipe surface and the inductive coil are increased, the 

induced coupling is adversely affected, requiring higher voltages to achieve power transfer. (This is further described in sec. 4.3.3.) The total heat flux between the heat pipe surface and the surface of the inductor (Q&

total ) is given by: 

Q&

total = Q&rad + Q&cond , (14)

where Q&

rad is the radiation component and Q&

cond is the conductive heat transfer load. The radiative 

component of the heat flux can be calculated using the Stefan-Boltzmann relationship with the surface 

emissivitiy (e), temperature (T), and area (A) of each of the relevant surfaces:

Q&

rad = σAi

(Thp

4 −Tind4 )

1/ εi + (Ai Ao)(1 / ε o −1) , (15)

where σ is the Boltzmann constant. For this analysis, the emissivity of Mo-44.5%Re is assumed 

to be 0.15 and that of copper is 0.75; a high value is assumed for conservative calculations. Figure 11 

illustrates the physical geometry used in this assessment. In addition, the static gas conduction heat 

flux component can be evaluated by the Fourier conduction heat transfer relationship for concentric 

cylinders,

Heat Pipe Centerline

Heat Pipe Internals

Heat Pipe Wall

Static Gap

RF Inductor Coil Inner Wall

do di

I

gap

Qcond

Qrad

Thp εi Ai

Tind εo Ao

.

.

Figure 11. Radio frequency inductor to heat pipe geometry.
31

Q&

cond = 2π Lck(Thp − Tind )

ln(do di) , (16)

where Lc is the heat pipe length under the RF coil and do and di are the outer and inner diameters, 

respectively.

Based on this approach, a spreadsheet model was developed to estimate the thermal losses to 

the inductive coil as a function of the gap width (Igap) for a maximum operating heat pipe temperature 

(1,373 K). The flow of water coolant through the inductive coil assembly is assumed to be sufficient 

to maintain an average coil temperature of 325 K. The complete spreadsheet is provided in appendix F 

with the results plotted in figure 12 (also includes estimated voltage breakdown values). As expected, 

small gap widths produce considerable heating rates on the inductive coil assembly (approaching that 

required by the actual heat pipe test). However, the heating rate drops off rapidly and becomes manageable 

at a gap width of 0.64 cm (0.25 in) with a value of ≈360 W. 

Gap Width Heat Pipe to RF Inductive Coil ID (cm)

Radiation Loss (W)

Gas Conduction Loss (W)

Total Heat Loss (W)

Voltage Breakdown (30%Ar)

Voltage Breakdown (5%Ar)

Voltage Breakdown (He)

Heat Loss to RF Coil (W)

Breakdown Voltage (V)

0 0.2 0.4 0.6 0.8 1 1.2 1.4

2,000

1,750

1,500

1,250

1,000

750

150

125

0

900

800

700

600

500

400

300

200

100

0

Figure 12. Heat pipe to RF inductive coil heat loss versus standoff distance (pressure = 76 torr).

An additional assessment is made to determine if the water coolant flow to the RF cluster (five 

inductive heater coils) is reasonable for the imposed heating rate. Total heating rate is the sum of thermal 

transport (heat pipe to RF coil) and the internal coil losses (RF efficiency). The internal RF coil losses 

are addressed in greater detail in the following section, but the appropriate expression is included here 

as a function of gap width (heat pipe to inductive coil):

Coil internal loss=723 (gap width)2+3,897 (gap width)2+3,725 , (17)
32

where the resulting loss is in watts and the gap width is in centimeters. The total lost power (or coil heating) is shown in figure 13 as a function of the heat pipe to coil gap width. The curve has a bucket shape 

with a wide, flat bottom centered at ≈0.4 cm. The selected gap width of 0.64 cm is very near this minimum, 

representing a conservative selection, since any increase in inductor losses (actual application influences) 

will tend to shift the bucket minimum to the right. These heating rates are converted to water flow requirements, assuming that an acceptable rise in water temperature (Tout –Tin) across the coil cluster is 50 K, 

producing an average temperature of 325 K: 

m& coolant = Q&

losses

cp (Tout − Tin ) , (18)

where m coolant is the water flow rate, cp is the specific heat of water, and Q&

losses is the total heat loss from 

the five inductive coil units (internal RF loss and thermal transport components). 

Flow Rate Assessed For: 

– RF Coil Cluster (Five Coils) 

– Water Temperature Rise of 50 K 

– Water Inlet Temperature of 300 K

Total Heating 

Internal Losses (Current)

Imposed Load (Heat Pipe)

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Gap Width Heat Pipe to RF Inductive Coil ID (cm)

Power Losses (Five-Coil Cluster)

16,000

14,000

12,000

10,000

8,000

6,000

4,000

2,000

0

Figure 13. Radio frequency inductive coil cluster (five coils) power loss due to internal RF losses 

and thermal transport from heat pipe.

Figure 14 illustrates the variation in flow rate as a function of heat pipe to RF coil gap width 

distance. The individual flow components are also indicated. At a coil gap width of 0.64 cm (0.25 in), 

a reasonable flow rate per cluster of ≈3.2 gal/min is required. This flow rate will be selected at the 

current design baseline.
33

Flow Rate Assessed For: 

– RF Coil Cluster (Five Coils) 

– Water Temperature Rise of 50 K 

– Water Inlet Temperature of 300 K

Total Coolant Flow

Internal Heating (Current)

Thermal Load (Heat Pipe)

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Gap Width Heat Pipe to RF Inductive Coil ID (cm)

Water Coolant Flow Rate (gpm)

6

5

4

3

2

1

0

Figure 14. Radio frequency inductive coil cluster (five coils) required coolant water flow rate.

4.3.3 Radio Frequency Inductor Coil Modeling

To estimate the RF system heating capability and to establish voltage, current, and frequency 

requirements, a quick assessment was performed using both one- and two-dimensional modeling by Goldstein.18 As identified in previous sections, it is important from a voltage breakdown and thermal loss 

standpoint to keep the inductive coil as far from the heat pipe evaporator section (or other grounded 

components) as possible. It is also critical to examine the effects of mutual inductance coupling on 

neighboring RF inductive coils (within the pentagonal array) and structural shunting components, such 

as the calorimeter assembly (positioned in close proximity). The modeling effort requires material 

resistivity data for the components of interest, including the Mo-Re heat pipe envelope and a thin layer 

of Na working fluid that is held along the interior surface of the heat pipe wall by the wick structure. 

These data are provided in appendix G. The only data readily available for Mo-Re was for a Mo-50%Re 

alloy. However, from an electrical standpoint, it should be nearly equivalent to the 44.5%-Re content 

material used in the heat pipe design. For all simulations, total RF power was estimated—coil losses 

and induced part power—using the 3-kW conditions. This assessment did not account for losses in 

interconnecting busswork, i.e., between the coils and to the power supplies.

An initial one-dimensional simulation was performed using the ELTA software package 

(Electro-Thermal Analysis marketed by BNV Corp.) to map out the general relationship between controlling parameters with a minimum of effort. This analysis assumes a 6.5-cm-long, single-turn inductive coil covering the evaporator which is at a temperature of 1,373 K. The goal of the initial effort was 

to determine the influence of applied RF on power transfer, required voltage, and efficiency for three 

offset gap widths (3.2, 6.4, and 9.6 mm). Results of the one-dimensional simulation are shown in 
34

figures 15–17. Initial estimates indicate that acceptable efficiency is reached above 10 kHz with a maximum at 20 kHz. (This range results from the hollow tube, which does not require the RF energy to 

penetrate deeply.) There is an ≈15% spread in efficiency between the 0.32 cm (0.125 in) and 0.96 cm 

(0.375 in) gap widths, with the smaller gap having higher efficiency. Required RF supply power is very 

high at low frequencies—expected due to low efficiency. The required supply power becomes approximately constant above a frequency of 10 kHz with minimal variation due to gap width. Coil voltage 

is notably sensitive to both gap width—gap becomes comparable with coil diameter—and supply 

frequency, producing higher voltage drops at large gaps and high frequency. A single coil is assumed 

for this analysis. In the actual application, a two- to three-turn coil would be employed. This design will 

result in a higher voltage for an equivalent power (voltage scales with inductor turns).

RF Power Supply Frequency (Hz)

1,000 10,000 100,000 1,000,000

RF Inductive Coil Efficiency (%)

For Five Inductive Coils in Series:

– Baseline 3-kW Power/Coil 

– Assumes Only Coil Thermal Losses 

– Assumes Single-Turn Coil 

– No Busswork Losses

3.2-mm Gap Efficiency

6.4-mm Gap Efficiency

9.6-mm Gap Efficiency

90

80

70

60

50

40

30

20

10

0

Figure 15. One-dimensional simulation results for inductive coil efficiency.

Overall, required power increases with increasing gap widths above 0.32 cm because the reduction in electrical efficiency is stronger than the higher thermal efficiency (material resistivity). However, 

the difference between 0.32- and 0.64-cm gaps is very small. In the actual test design, it is expected 

that the maximum efficiency will be shifted to slightly higher frequency due to electromagnetic effects. 

Required power supply output should be in the range of 20 to 25 kW at a properly tuned frequency 

(in the 25- to 40-kHz range) with a reasonable gap width set to 0.64 cm. The internal coil power loss 

as a function of gap width (eq. (17)) was established by curve fitting the difference in RF supply output power and the power induced in the heat pipe at 30 kHz for the gaps identified in figure 16. To 

accomplish this, the power supplies should be capable of providing an output frequency up to 50 kHz at 

50 kW with an output voltage of 400 to 500 V. The design point is approximately one-half of the full 

output power so that overall thermal losses can easily be overcome. In addition, the supplies must be
35

RF Power Supply Frequency (Hz)

RF Supply Required Power (W)

1,000 10,000 100,000 1,000,000

3.2-mm Gap Power

6.4-mm Gap Power

9.6-mm Gap Power

For Five Inductive Coils in Series:

 – Baseline 3-kW Power/Coil 

 – Assumes Only Coil Thermal Losses 

 – Assumes Single-Turn Coil 

 – No Busswork Losses

60,000

50,000

40,000

30,000

20,000

10,000

0

Figure 16. One-dimensional simulation results RF power supply output requirement (five coils).

RF Power Supply Frequency (Hz)

Voltage Drop Across All RF Coils (V)

1,000 10,000 100,000 1,000,000

For Five Inductive Coils in Series: 

 – Baseline 3-kW Power/Coil 

 – Assumes Only Coil Thermal Losses 

 – Assumes Single-Turn Coil 

 – No Busswork Losses

3.2-mm Gap Power

6.4-mm Gap Power

9.6-mm Gap Power

600

500

400

300

200

100

0

Figure 17. One-dimensional simulation results—coil assembly required voltage (five coils).
36

equipped with variable output transformers with turn ratios of 2:1, 3:1, and 4:1 to adjust the required output voltage to the test chamber clusters. For the final setup of the 3-kW case (used in test chamber clusters 

1 and 2), it is expected that the clusters will make use of three-turn inductive coils. To provide for the various RF power levels within test chamber cluster 3, it is expected that a two-turn inductor will be used for 

the single 1,000-W coil, a three-turn inductor for the two 2,000-W coils, and a four-turn inductor for the 

two 4,000-W coils. Appendix H provides a spreadsheet summarizing the results of the one-dimensional 

simulations.

After establishing the general operating parameters using the one-dimensional model, a twodimensional model was generated (using Flux2D® software marketed by CEDRAT Technologies) to 

examine the performance of a three-turn inductor for the 3-kW RF coil case. This model also examined 

shunting between the inductive coil and the heat pipe calorimeter, which is in close proximity, and the 

mutual inductance shared with neighboring coils. A total of four cases were modeled: 

(1) Inductor with no neighboring coils; flux concentrator applied for shielding.

(2) Inductor with no neighboring coils; no concentrator.

(3) Inductor with a 3.5-in distance from centerline to centerline; flux concentrator applied for 

shielding.

(4) Inductor with a 3.5-in distance from centerline to centerline; no flux concentrator.

Results for each of these simulation cases are shown in figures 18–21, with tabulated data provided 

in appendix I. The benefit of using a flux concentrator is immediately evident. Magnetic field lines are 

contained primarily to the concentrator and heat pipe (figs. 18 and 20), providing more uniform heating 

across the heat pipe evaporator and lower overall power consumption. Without a concentrator, the power 

density at the heat pipe evaporator will be shifted to the back of the inductor due to shunting from the calorimeter (fig. 19). This leads to nonuniform loading along the heat pipe evaporator. With the concentrator, 

the power density distribution is nearly uniform and symmetric. In addition, coupling to the calorimeter is 

significantly reduced, dropping from 70 W without a concentrator to 6 W with a concentrator. (See tabulated data in app. I.) The concentrator also limits mutual induction or “cross talk” between the coils in the 

areas where the inductors are close (figs. 20 and 21).

In summary, flux concentrators provide sufficient shielding to render azimuthal variation 

of power in the evaporator and losses due to the calorimeter negligible. Without concentrators, the 

induction coil would need to be moved farther from the calorimeter and the inductors spread farther 

apart. Another advantage of applying the magnetic flux controller is that the coil current demand will 

be reduced (≈14%). This means that the losses in the busswork and other supplying circuitry will be 

significantly reduced (by ≈30%, proportional to I2 heating), allowing more power to be available 

to overcome uncertainty in overall thermal losses. A heat pipe to inductor coil gap width on the order 

of 0.64 cm (0.25 in) is suitable from a thermal, voltage breakdown and RF power perspective and will be 

established as the current baseline.
37

Calorimeter

RF Coils (Three Turns)

Flux Concentrator

Heat Pipe Wall and Sodium Heat Pipe Centerline

Figure 18. Radio frequency coil with flux concentrator in open space.

Figure 19. Radio frequency coil without flux concentrator in open space.
38

Figure 20. Radio frequency coil with flux concentrator; close to neighboring inductor.

Figure 21. Radio frequency coil without flux concentrator; close to neighbor inductor.

4.4 Overall Radio Frequency Power System Layout

The general RF power subsystem hardware configuration is illustrated in figure 22. This layout 

is composed of several primary subsystems, including the following:

• Radio frequency inductive coil assemblies (clusters) with vacuum-compatible feedthroughs.

• Radio frequency power distribution busswork connecting the power supplies to the chamber.

• Radio frequency power supplies. 

• Water cooling for both the power supplies and the RF inductive coil assemblies.
39

Facility Water

System

RF Supply Water Cooling

Recirculation System

4655 Int

Supply

4655 Int

Supply

FS

FS

TC

FM TC

FM

TC

P

P

P

L P

L

Facility Disconnect

480 VAC 3-P 

Power Feed to RF Supplies Cluster No. 1

Cluster No. 2

Cluster No. 3

Cluster No. 4

RF Power Output

RF Power Return

Step-Down

Transformer

RF Power Sensor at Each Coil

RF Generator Units (x3)

– 50 kW

– 400 to 500 V

– 20 to 50 kHz

RF Generator Units

– 15 kW

– 250 V

– 20 to 50 kHz

Thermocouple

Pressure

Flow Meter

Hand Valve

Relief Valve

Flow Switch

Pump

Figure 22. General RF power subsystem hardware layout.

There is a total of four RF coil assemblies, three of which are clusters (composed of five series 

connected coils) and one as a single element. All RF assemblies are positioned within the environmental 

test chamber and supported structurally by the RF power feed configuration. Total power flow through 

the inductor assemblies will be measured at the power supply; however, an attempt will be made to 

incorporate a power sensor at each RF inductive coil—the type and geometry are to be determined. The 

final design and fabrication of these complete RF coil assemblies with connecting busswork will be pursued through an open solicitation with a procurement specification discussed in the following section. 

The RF coil assemblies are connected to the power supplies using a buss network and variable transformers to step down the power supply natural output voltage (typically 400 to 500 V) to 

a level sufficiently low to prevent a voltage breakdown condition. Each of the RF clusters is connected to 

a separate solid state RF power supply; three of these units are rated at 50 kW (powering clusters 1, 2, 

and 3) and one unit is rated at 15 kW (powering cluster 4). All power supplies and RF inductive coil 

assemblies require cooling that is provided by a closed-loop water circulation system that exchanges 

heat energy with the facility water system. This closed-loop water system provides a well-controlled 
40

interface to the facility, necessary for long-term reliable operation of the test equipment. This eliminates 

the possibility of transporting debris, etc. into the RF cooling system. It also provides for a more graceful 

shutdown should the facility system lose power. (There is excess cooling capacity in the circulation loop 

that can absorb heat as the RF supplies are shut down.) The typical temperature of the facility water system 

varies, depending on time of year. The primary storage tank is placed outside the building and will require 

a small water-cooling tower cell for the extended operations expected with this project. 

Instrumentation will be provided to monitor the temperature and flow of the facility and recirculation cooling loops. The power supplies are equipped with internal detection systems to monitor 

the coolant flow to the RF inductive coil assemblies. Temperature measurements will make use of type 

K thermocouple probes. Flow rates will be measured using either turbine or mass flow meters (cost 

dependent). Flow switches will be used in lines that only require a flow indication, e.g., where the actual 

magnitude of the flow rate is unimportant. For instance, a flow switch may be incorporated in the facility 

feed lines. The RF power supplies are outfitted with control boards that provide for zero to 10-V input 

and output signals to control/monitor voltage, current, frequency, and power. This allows for remote 

computer-based control and monitoring.

4.5 General Procurement Specifications

Procurement specifications have been generated to cover several of the primary components necessary to complete the assembly and operation of the proposed heat pipe test system. These procurements 

will be issued by MSFC using full and open competition. The final evaluation of vendor quotes will be 

based on the lowest cost technically acceptable approach. The primary requirement areas include the 

following: 

(1) Design and fabrication of RF inductive coil assemblies and power feed systems. This will 

include all four RF clusters—each test chamber section—with busswork to connect each RF coil cluster 

to a power supply. This requirement also includes instrumentation to monitor overall performance, such 

as sensors strategically positioned to monitor individual coil power. Appendix J contains the detailed RF 

inductive coil assembly procurement specification. Typical vendor delivery time is 12 to 18 wk.

(2) Closed-loop recirculation system to provide coolant to all RF power supplies and their inductive coil assemblies. The system will be a commercial off-the-shelf item, such as the Compact WRS (marketed by Inductoheat) which includes a pallet-mounted heat exchanger, pump, storage tank, stainless steel 

lines/tank, and cooling control relays. It is designed for long-term continuous operation. Typical vendor 

delivery time is 6 to 8 wk. 

(3) Small RF power supply with 15 kW at zero to 60 kHz output to enable testing of the highpower heat pipe (cluster 4). The unit is a commercial off-the-shelf item such as the solid state Flexitune 

15+ (marketed by Inductoheat) capable of 24/7 operation at 100% duty cycle with an internal control 

board providing zero to 10-V input and output signals allowing for remote monitoring and control. Typical 

vendor delivery time is 8 to 10 wk. 

(4) A small cooling tower cell will be required to maintain cooling of the primary water supply. 

A unit on the order of 75 to 90 kW is sufficient to maintain a constant coolant temperature. Commercial 
41

off-the-shelf items such as the TC-30-230, a 30-ton sheet steel unit, or a TC-45-F-230, a 45-ton fiberglass 

unit, both marketed by Advantage Engineering Inc. are suitable for this application. Both provide up to 

90 gpm with a processed water temperature of 300 to 310 K. Typical vendor delivery time is 2 to 4 wk.

(5) Plumbing components such as valves, including hand and remote operated, fittings, gauges, 

and structural material for setup and operation of the RF cooling circuit components. These will be balltype valves making use of either compression or pipe fittings. In addition, pressure-relief valves are 

required. Valve material will be stainless steel with soft goods compatible with water service and includes 

appropriated flow and temperature sensors. Typical vendor delivery is 2 to 4 wk.

(6) Part fabrication and assembly at machine shops, as required to complete system. 

(7) Radio frequency power sensors/meters includes instrumentation to monitor overall performance, such as sensors strategically positioned to monitor individual coil power, structural supports, etc. 

to route busswork, and additional feedthroughs for instrumentation—temperature, power sensors, and 

optical. These are typically off the shelf with delivery ranging from 4 to 8 wk.
42

5. PURIFIED INERT GAS CONDITIONING SYSTEM

The ambient oxygen concentration is critical to the lifetime of refractory metals operated at 

increased temperature for a relatively long time. For the proposed testing, the refractory metal components 

will be operated in a low-pressure, ultra-high-purity (UHP) noble gas environment (Ar and He mixed 

to a predetermined partial pressure of each gas) so that the thermal coupling between the heat pipe and 

calorimeter can be controlled. To achieve the desired gas purity, successive dilutions and pumpdown of 

the system are required. The pumpdown procedure will also include an initial bake-out of the system at 

≈525 K to drive off water vapor (and other volatiles) from the gas lines, vacuum chamber, and test components. A generally accepted vacuum level for testing Mo-Re alloys is in the 10–6 torr range. Given a direct 

pumpdown from air to 10–6 torr, the oxygen concentration is 0.28 ppb in the vacuum test environment. 

Hence, the target maximum oxygen concentration in the test chambers will be 0.28 ppb at the desired 

operating pressure of ≈76 torr (the set pressure used in calculating gas conditioning requirements).

5.1 Sizing and Performance

As discussed in report NASA/TP—2010–216435,

1 a series of calculations was performed to assess 

the oxygen concentration in the heat pipe test chambers over successive dilutions with UHP fill gas, 

assuming an initial oxygen concentration of 209,500 ppm in the air filling the test chambers (20.95% oxygen by volume). Calculations were performed assuming three cylindrical test chambers having approximate dimensions of a 24-in diameter (23.625 in ID) by 36-in length (total volume 48,000 in3 (0.78 m3) 

for all three chambers). These test chambers will be used for the 1-, 2-, 3-, and 4-kW heat pipe tests. An 

additional smaller chamber will be constructed for the 5-kW heat pipe test. All heat pipe life testing will 

be performed with a low-pressure fill of mixed He and Ar in the test chambers. The UHP He and Ar, purchased from Sexton Supply, Huntsville, AL, have a guaranteed minimum purity of 99.999%. The UHP He 

has a maximum oxygen content of 3 ppmv; UHP Ar has a maximum oxygen content of 1 ppmv.

To obtain the oxygen concentrations shown in figure 23, the test chambers are first pumped 

from atmospheric pressure to 10–3 torr, e.g., operating only the roughing pump connected to the test 

chambers. Note that the plots correspond to a 76-torr chamber pressure, the approximate pressure at 

which the heat pipe life tests will be performed. Initially, pumping the chamber to a pressure below the 

desired operating pressure results in a drastic reduction in the oxygen concentration from 209,500 ppm to 

0.3 ppm at 10–3 torr. Direct pumpdown to only 76 torr reduced the oxygen concentration to approximately 2 × 104 ppm, as shown in figure 23 prior to fill with UHP gas at 76 torr. After this initial pumpdown, the chambers were backfilled to 76 torr of UHP He or Ar and again pumped down to 10–3 torr 

to further reduce the oxygen concentration in the test chambers. After just two dilutions with UHP gas 

(He or Ar), the test chambers reach the minimum purity level achievable, given the purity of the supply gas. While these calculations ignore any additional contamination from impurities in the lines and 

test chamber, they do provide an ultimate baseline for the minimum achievable oxygen concentration for a given fill gas and operating pressure without additional gas purification. The test procedure

will also include an initial bake-out of the system (under vacuum), which will assist in driving out 
43

0 1 2 3 4 0 1 2 3 4

O2 (ppmv) at 76 torr

O2 (ppmv) at 76 torr

Number of Dilutions Number of Dilutions

1×105

1×104

1×103

1×102

1×101

1×10

1×10–1

1×10–2

1×105

1×104

1×103

1×102

1×101

1×10

1×10–1

1×10–2

(a) (b)

Figure 23. Oxygen concentration (ppmv) in the heat pipe test chambers following 

successive dilutions with (a) UHP He or (b) UHP Ar fill gas.

volatile impurities, primarily water if the system is thoroughly cleaned and degreased, from the test 

components and fill lines to reduce additional impurities in the system. For additional discussion on 

system bake-out and the suggested test procedure, refer to sections 5.1.2 and 5.1.4.

The 5-kW heat pipe life test will be performed in a smaller test chamber having an approximate 

diameter of 12 in and a length of ≈36 in, yielding a total volume of 1,296 in3 (0.021 m3). Because the 

same supply gases will be used in this chamber, the ultimate minimum purity level of the fill gas will be 

limited to the same levels shown in figure 23 (0.3 ppm O2 for He, 0.1 ppm O2 for Ar) without additional 

gas purification.

As discussed in NASA/TP—2010–216435 (Design of a Refractory Metal Life Test Heat Pipe and 

Calorimeter, sec. 4.2.1), the gas mixture will be selected to achieve the desired gas conductivity across the 

gap between the heat pipe condenser and the calorimeter.1 The gas mixture selected for the lower power 

(1–4 kW) heat pipe tests was He-32%Ar (molar fraction). The increased heat flux for the 5-kW test and 

the minimum gap width established for fabrication purposes (0.020 in) requires that the Ar content in the 

gas mixture be reduced to 6% (molar fraction) for this test. The partial pressure of each gas in the mixture 

is given by the molar fraction multiplied by the total gas pressure. For instance, for a total pressure of 

100 psig and a 32% Ar mix, the partial pressure of Ar will be 32 psig and the partial pressure of He will 

be 68 psig. Partial pressures will be used to establish the proper gas ratios in the He/Ar mixing procedure.

5.1.1 Gas Purification

Additional purification of the UHP gases will be required to meet the required oxygen concentration of <0.28 ppb (comparable to 10–6 torr) for testing Mo-Re alloys. To accomplish this purification, an 

SAES ambient temperature microtorr gas purifier will be used to clean the incoming gases and an SAES 

monotorr point-of-use purifier will be incorporated in the test chamber recirculation system. Figure 24 

illustrates a conceptual layout of the test chamber gas purification system.
44

Turbo Vacuum Pump

and ISO Valve

Test Chamber With 

Heat Pipes

RGA

TC

FM TC

Set 20 psia

G-RV5 Set 120 psig

G-RV2

Set 20 psia

G-RV3

G-HOV10

G-HOV11

G-HOV12

G-HOV14

G-HOV15

G-HOV9

G-HOV7

G-HOV8

G-HOV6 G-HOV4

G-HOV3

G-HOV1

G-HOV2 G-HOR2

G-HOR1

G-HOV5

G-HOV17

G-HOV16

G-HOV13

G-TC1 G-RV4

Set 20 psia

G-RV1

Set 120 psig 0–1,000

Torr

0–150

psi 0–4,000 psi

0–150

psi

0–150 psi

0–4,000 psi Gas Pump

1⁄2-in Tubing 1⁄4-in Tubing

Bypass Valve

Recirculating

Flow Path

Monotorr

Purifier

Bleed Valves

Gas Mixture

Bottle

Duplicate Fill

System for Small

Test Chamber

Microtorr

Purifier

Helium Supply

K-Bottle

Argon

Supply

K-Bottle

Evacuation

Port

HOV: Hand-Operated Valve

HOR: Hand-Operated Regulator

RV: Pressure Relief Valve

RGA: Residual Gas Analyzer

TC: Thermocouple

FM: Flow Meter

Identifier Key:

Figure 24. Test chamber gas mixture and purification system.

The design of the gas mixture and purification system is such that the recirculating gas system 

can be fully isolated from the gas mixing/fill system using the hand valve G-HOV9, allowing the gas 

mixture bottle to be charged with an appropriate He/Ar mixture prior to test chamber fill. The small test 

chamber fill system will be connected to the main fill system just after the microtorr purifier (G-HOV5 

opens the fill line to this chamber), such that the same He and Ar supply bottles and initial purification 

system may be used for both systems. The premixed fill gas can be mixed to the desired partial pressures 

of He and Ar to meet both test requirements using separate gas mixture bottles and recirculating gas 

systems.

The microtorr purifier, which has an advertised performance for purifying both He and Ar 

to a final oxygen concentration of 1 ppb, will be employed on the inlet flow line. Over multiple gas 

dilution cycles, this new value of oxygen concentration in both He and Ar produces the trends shown 

in figure 25. After only two dilution cycles, starting from atmospheric pressure, the final oxygen impurity 

concentration is just 0.1 ppb at a 76-torr test chamber pressure. This condition meets the acceptable oxygen concentration (≈0.28 ppb) for testing Mo-Re alloys. 

The valves selected for the inlet low-temperature, high-pressure side of the gas purification system will be stainless steel Swagelock SS-4H bellows valves having a 1⁄4-in tube fitting on both the inlet 

and outlet ports. These valves have a maximum temperature rating of 315 oC (590 K) and a maximum 

pressure rating of 1,000 psig. The low-pressure recirculation system will use 1⁄2-in stainless steel tubing 

and all valves will be Swagelock SS-8BW-V47. These valves use VCR fittings and are less prone to leakage, making them appropriate for long-term use on the recirculation system. Details of all system components are provided in appendix K.
45

0 1 2 3 4

O2 (ppmv) at 76 torr

Number of Dilutions

1×105

1×104

1×103

1×102

1×101

1×10

1×10–1

1×10–2

1×10–3

1×10–4

1×10–5

Figure 25. Final oxygen concentration in the heat pipe test chambers with UHP gas, purified using 

an inline SAES microtorr ambient gas purifier to 1 ppb O2 prior to chamber inlet.

The “gas mixture bottle” will be limited to a maximum pressure of 100 psi (0.69 MPa) to prevent 

overpressurizing any system components. The microtorr purifier located just upstream of the gas mixture 

bottle has a pressure limit of 150 psi and pressure relief valves on the fill lines (G-RV1 and G-RV2) will 

be set to 120 psi. The mixed gas will be used to fill the three larger test chambers at a pressure of ≈76 torr 

(0.10 MPa). The volume of a single test chamber (ID=23.625 in, L=36 in) is given by:

V=π * (23.625 in)2 * 36 in/4=15,781 in3=0.259 m3 . (19)

Hence, the total volume of the three test chambers that are to be filled by the gas mixture is 

0.78 m3. Ignoring any volume of gas trapped in the flow lines and applying the ideal gas law, pV=nRT, 

the desired volume of the gas mixture bottle can be obtained:

pchamberVchamber=pbottleVbottle

(10,000 Pa) (0.78 m3)=(690,000 Pa) V2

→V2=0.011 m3 . (20)

To allow for multiple dilutions of the fill gas in the test chambers, the gas mixture bottle should 

have sufficient capacity for at least three complete fills of the test chambers at 76 torr (0.10 MPa). 

Hence, the total volume of the gas mixture bottle for the three large test chambers should have a minimum volume of 0.033 m3, which corresponds to an approximate 34-L water capacity. A steel H-bottle 

(DOT specification 3AA), rated to a maximum pressure of 2,015 psi, has an approximate volume of 

43.4 L of water, meeting the minimum requirements for the mixed gas cylinder. Specifications on this 

cylinder are included in appendix K.
46

To maintain the purity level of the test chamber environment during test operations, an SAES 

monotorr point-of-use purifier with a recirculation pump will be incorporated in the recirculating gas 

loop. During chamber fill, the monotorr will be isolated from the flow path. After sufficient dilutions 

have been performed for the test chamber gas and the desired pressure achieved, valve G-HOV9, which 

separates the gas fill and recirculating systems, should be closed. An additional series of valves is provided to allow recirculation of the gas in the test chambers with or without incorporation of the monotorr purifier (by closing G-HOV13 and G-HOV14 and opening G-HOV15). The test chambers may be 

bypassed by closing G-HOV10 and G-HOV11 and opening G-HOV12. 

Within the monotorr purifier, getter materials are used to irreversibly trap gaseous impurity molecules. These impurities are captured on the surface of the materials and, upon heating, diffuse into the 

bulk of the getter. Note that if the filters inside the monotorr are inadvertently exposed to air, the filters 

will be immediately filled, and the unit will require extensive overhaul before it can be applied again 

in the purification loop. The internal heater within the monotorr device has an operating temperature 

of 400 oC; as a result, gas exiting the monotorr will have an elevated temperature up to as much as 

400 oC, which may require that the lines be cooled by water circulation downstream of the monotorr. For 

this reason, the gas pump is located upstream of the monotorr purifier to prevent overheating of the pump 

internals. When used with Ar or He, the monotorr purifier can remove molecules of H2O, O2, H2, CO, 

CO2, N2, and hydrocarbons. The performance of the purifier is dependent on the pumping speed at which 

it is operated. Table 10 provides a summary of the performance guarantee for the SAES monotorr phase II 

3000 for rare gases. 

Table 10. SAES monotorr phase II 3000 performance guarantee for rare gases.

Impurity

Pumping Rate

0–20 slpm 20–50 slpm

O2 <1 ppb <1 ppb

H2O <1 ppb <1 ppb

CO <1 ppb <1 ppb

CO2 <1 ppb <1 ppb

N2 <1 ppb <10 ppb

H2 <1 ppb <10 ppb

CH4 <1 ppb <10 ppb

The gas pump on the recirculating gas purification system will be specified to provide a volumetric 

flow rate of up to 20 slpm at a system pressure of ≈0.1 MPa. At this operating pressure, a gas temperature 

of ≈300 K, this corresponds to ≈207 alpm (actual liters per minute). To convert this volumetric flow rate 

into a mass flow rate for the He/Ar mixture, the density of the mixture must be determined at the specified 

test conditions. Assuming no particle interactions in the gas mixture, the mixture density 

is given by

ρmix = ρArVAr + ρHeVHe

VAr +VHe

 . , (21)
47

The density of pure He at 300 K and 0.1 MPa is 0.016 kg/m3; that of pure Ar is 0.16 kg/m3. For the 

He-32%Ar mixture, the calculated mixture density is 0.062 kg/m3. Given this density, the maximum mass 

flow rate, corresponding to a volumetric flow rate of 20 slpm, of the gas mixture is 0.2 g/s. To put this rate 

into perspective, the total mass of gas in the three test chambers at 0.1 MPa is (0.78 m3)*(0.062 kg/m3)= 

≈50 g. At the maximum mass flow rate of 0.2 g/s, it would require 250 s to fully cycle the gas in the test 

chambers. The calculated density of the He-6%Ar mixture is 0.025 kg/ m3. This lower density results in 

a lower mass flow rate of 0.086 g/s at the same volumetric flow rate of 20 slpm.

Pressure relief valves have been incorporated throughout the gas purification loop for safety 

purposes. On the high-pressure side of the fill system, all relief valves (G-RV1, 2, 3) are set to 120 psig 

to prevent overpressure of the microtorr purifier (max 150 psig). All valves that will be used in this 

purification system are rated to a maximum pressure of 1,000 psig. The monotorr purifier is limited 

to a maximum of 100 psig, but the limiting pressure requirement will be that which can be placed on 

the test chamber. Because the chamber is designed to operate at subatmospheric pressure (14.7 psi), the 

relief valves on the recirculating lines (G-RV4, 5, 6) will be set to 20 psia. An additional relief valve 

is included within the recirculation loop to prevent overpressurizing the test chambers. 

A rough calculation of the pressure drop around the recirculating gas loop was performed to 

ensure that it would not affect system operation. For this purpose, it is assumed that the length of tubing from the test chamber to the monotorr inlet was ≈ 4 ft (≈1.2 m) and from the monotorr exit back to 

the test chambers was ≈ 4 ft (≈1.2 m). The simplified flow loop for the pressure calculation is shown 

in figure 26. 

The stated maximum pressure drop across the monotorr is <10 psi at the maximum flow rate 

of 50 slpm and the maximum rated pressure of 100 psig. Assuming that the pressure drop scales linearly 

with flow rate, as in a straight section of pipe, the pressure drop reduces to <4 psi, also at the maximum 

rated pressure of 100 psig. Recall that the recirculating loop will be operated at a pressure of ≈1.5 psig 

(≈76 torr). Again, assuming that the pressure drop will scale linearly with the system pressure, this reduces 

the pressure drop across the monotorr to ≈0.06 psig (414 Pa). Loss coefficients for additional system components are summarized in table 11.

The loop pressure drop is given by

Δp = ρ f L

D

v

2

2 , (22)
48

Test Chamber With

Heat Pipes

Monotorr

Purifier

G

P

Figure 26. Simplified test loop for pressure drop calculation.

Table 11. Loss coefficients for system pressure drop calculation.

Component

Loss Coefficient or 

Equivalent Length

Chamber exit/pipe entrance Kent = 0.5

Elbow, 90° Le /D= 30

Tee, flowthrough branch Le /D= 60

Valves, bellows (approximated 

 as globe valves)

Le /D= 350

Chamber entrance/pipe exit Kexit = 1

*Note: Loss coefficients provided in Fox and McDonald.19

where the friction factor for laminar flow (Reynolds number (Re) <2,300) is

f = 64

Re , (23)

and 

Re = ρv D

μ . (24)

To include the major and minor head losses in the pressure drop, the relationship

hl = K v 2

2 or hl = f LeD

v 2

2 , (25)
49

and

hl = Δp

ρ , (26)

where Le /D corresponds to the equivalent length of straight pipe, should be used. For the He-32%Ar 

mixture, having density 0.062 kg/m3, viscosity 2.35×10–5 kg/m-s, and a maximum flow velocity of 

27.1 m/s (at 20 slpm, 0.1 MPa, 300 K), the pressure drop becomes ≈25 torr (3,400 Pa) for 1⁄2-in tubing. 

This result neglects any possible pressure drop across the gas pump, as it has not yet been selected for 

this application. The pressure drop along the same path for the He-6%Ar mixture will be reduced due 

to ≈23 torr (3,100 Pa) due to the reduced gas density.

5.1.2 System Bake-Out

The test procedure will include an initial bake-out of the system (under vacuum), which will 

assist in driving out volatile impurities, primarily water if the system is clean and degreased, from the 

test components and fill lines to reduce additional impurities in the system. An endothermic process, 

desorption (or outgassing) is accelerated by increased temperature. The rate at which gas appears 

to emanate from a surface is referred to as the outgassing constant. This value is usually given in torrL/s-cm2. It is advisable to begin a degassing program by first pumping down the system at room temperature to remove physically adsorbed water before commencing the baking cycle. If heat is applied at 

atmospheric pressure, it could result in activated chemisorption of physically adsorbed gas, which would 

require a prolonged heating cycle to remove. 

In Vacuum Technology, Roth summarized several works that have determined the outgassing 

rate for stainless steel under various conditions.20 Edwards reports that the upper bound for the outgassing rate of low surface area metals, such as stainless steel, is 1.7×10–5 torr-L/s-cm2.

21 As the baking/

air-exposure cycle is repeated, Odaka et al. report that a constant minimum outgassing rate for stainless steel 316L is achieved in just a few cycles.22 The minimum rate measured by Odaka et al. was 

1×10–13 torr-L/s-cm2. The outgassing rate for stainless steel can also vary based on the surface treatment in addition to the temperature cycling history. Data compiled by Roth indicate the approximate 

ranges of outgassing rates for stainless steel that are summarized in table 12.20 Note that untreated, 

degreased, and polished samples were outgassed over 4 to 8 hr of pumping. The baked condition 

experienced the baking cycle summarized in table 12.

Table 12. Approximate outgassing rates of stainless steel as reported in Roth.20

Material Treatment

Outgassing Rate (qD)

(torr-L/s-cm2)

Untreated ≈10–7–10–8

Degreased ≈1–7×10–9

Polished ≈10–9–5×10–11

Baked

 High range qD: 24 hr @ 300 °C

 Mid range qD: 100 hr @ 400 °C

 Low range qD: 3 hr @ 1,000 °C

(bake-out cycles applied sequentially)

≈5×10–11–10–14
50

The three large test chambers alone have an approximate surface area given by:

SA = 3[πDH+2 πD2/4] 

= 3 π[24 in × 36 in+(24 in) 2/2] 

= 10,800 in2=7 × 104 cm2 . (27)

At a chamber pressure of 76 torr, the maximum volume that will emanate from the untreated chamber 

surfaces would be approximately 

(10–7 torr-L/s-cm2)×(7×104)/76 torr=9.2×10–5 L/s . (28)

If bake-out is performed at a pressure of 10–6 torr, the volumetric outgassing rate becomes 

7×103 L/s, but with significantly lower vapor density. Increasing the chamber wall temperature decreases 

the outgassing rate to ≈10–11 to 10–14 torr-L/s-cm2, depending on the temperature and elapsed time for the 

bake-out. At 10–6 torr, this corresponds to approximately 0.7 to 7×10–4 L/s. To interpret these outgassing 

rates, the density of the water vapor at the pressure and temperature for each condition must be taken into 

account to determine the mass of water vapor emanating from the chamber surface per unit time. These 

results are summarized in table 13.

Table 13. Approximate volume and mass of water vapor emanating from the three large test 

chamber surfaces per unit time.

Test Condition

Density of 

Water Vapor 

(g/m3)

Volumetric 

Outgassing Rate 

(L/s)

Mass of Water Vapor Leaving 

Chamber Surfaces per Time 

(g/s)

300 K, 76 torr 72 9×10–5 6.5×10–6

300 K, 10–6 torr 9.4×10–7 7×103 6.6×10–6

570 K, 10–6 torr 4.9×10–7 7×10–1 (low range)

7×10–4 (high range)

3.4×10–10

3.4×10–13

Note: Density assumes that water vapor can be represented as an ideal gas.

Table 13 illustrates the significant reduction in the mass of water vapor leaving the surfaces of the 

test chamber after extended baking at elevated temperature. Therefore, test procedures will include a bakeout of the test chamber and all flow lines at 250–300 oC for up to 24 hr prior to chamber fill with UHP gas.

5.1.3 Gas Sampling: Residual Gas Analyzer Mass Spectrometer

Residual gases in a vacuum chamber derive from the original gas content of the chamber, gas 

emission from the chamber walls or from hardware in the chamber, or leaks from the outside of the chamber. To determine the actual gas content in the heat pipe test chambers and to assess the effectiveness of 

the bake-out and gas purification processes, a residual gas analyzer (RGA) will be employed to verify the 

partial pressure of various gaseous components in the test chambers throughout test. 
51

A Dycor Dymaxion Mass Spectrometer made by Ametek Process Instruments (model DM100M) 

will be employed in the heat pipe life test system. All mass spectrometers measure a current that is either 

directly due to or is derived from ions produced in a source and selected by some mass to charge selection device. The Dycor Dymaxion Mass Spectrometer utilizes a quadrupole mass analyzer. A hot filament in the RGA is used to create electrons of a suitable energy. These electrons then generate a stream 

of ions, where the rate depends on the pressure, temperature, and species of the individual molecules. 

The stream of ions is then electrostatically focused toward the mass filter. A quadropole mass filter 

consists of four metal rods having a time-varying electrical voltage applied, selected to only ions of 

a particular mass to enter along the axis and to pass through to the opposite end. After passing through 

the mass filter, ions are focused toward a Faraday cup and the current is measured using a highly sensitive ammeter. The resulting signal is proportional to the partial pressure of the ion species that was 

passed by the mass filter. Because the current produced by residual gases is very small, a sensitive signal amplifier is also required in the system to detect very small partial pressures of the various gases 

that may be present. The Dycor DM100M can be used to detect masses up to 100 amu at a minimum 

detectable partial pressure of 5×10–12 torr.

A sampling system serves as a bridge between the sample environment and the vacuum environment of the mass spectrometer (10–4 to 10–9 torr). If the sample environment is at or below 10–4 torr, 

the sample can be analyzed directly. Higher pressure sample environments will require a pressure 

reduction sampling system. In these instances, a high-conductance inlet (HCI) can be fitted to the sample 

chamber. The HCI consists of a length of tubing that is installed between the sample chamber and the mass 

spectrometer inlet. When the sample chamber is between 10–4 torr and atmospheric pressure, an aperture 

inlet (AI) is employed. The AI consists of a small aperture within a valve that connects the sampling system to the vacuum environment. The high-conductance valve is first closed tightly before slowly opening 

the AI to evacuate the bypass arm. The aperture size and chamber pressure are directly related through 

eq. (29):

Aperture size (μ)=50/(sample chamber pressure (torr)) . (29)

Hence, for a maximum sample chamber pressure of 100 torr, the recommended aperture size 

is 5 μ. The Dycor User’s Manual also indicates that, for “atmospheric sampling” (10 –103 torr), the 

RGA will require a capillary tube inlet (CTI) of varying length to accommodate the pressure of the 

application. 

The Dycor DM100M is additionally equipped with RS-232 and RS-485 communications ports 

such that the RGA can communicate with the experimental system over a network. This feature allows 

for the RGA to be used for both the large test chambers and the smaller test chamber, as shown in figure 27. Solenoid-operated valves, which can be operated remotely, are included to allow automatically switching between the test chambers to sample the fill gas with the RGA. With both G-SOV1 and 

G-SOV2 closed, the RGA can be initialized and pumped down. To sample the gas content of the large 

test chamber, G-SOV1 would be opened and the RGA allowed to run for a predetermined sampling time. 

To switch over to the small test chamber, G-SOV1 would first be closed, the RGA allowed to operate 

for a period of time to clear the lines, and then G-SOV2 would be opened. To minimize both the cycle 

time and amount of removed chamber gas, the solenoid valves shall be located as close to the RGA 

as physically possible. The control system will include times at which the RGA should switch between 
52

Large Test Chambers, Common Gas Fill

Small Test Chamber

G-SOV1 G-SOV2

RGA

Figure 27. Residual gas analyzer sampling system.

test chambers, and will include engineering controls should the gas in the test chambers be out of the 

required bounds, e.g., high oxygen concentration.

5.1.4 Gas Fill and Test Procedure

A general procedure for initial pump-down, bake-out, and gas fill of the heat pipe life test chambers is provided below. Before performing bake-out operations, the test chambers and gas system components should be connected to a low-flow, high-purity inert Ar or N purge for 24 hr. This purge will assist 

the bake-out process by both sweeping out residual atmosphere and drying the internal surfaces of the 

test hardware. The purge should be shut down prior to initiating any of the following procedures. These 

general procedures are written for operation of a single heat pipe test chamber system. For additional test 

chambers, the procedures would be identical, with the exception of component identifier tags. All test 

chamber systems would share a common inert gas supply and microtorr purifier.

5.1.4.1 Gas System Bake-Out. The procedure for gas system bake-out includes the following:

(1) Check that all hand valves are closed prior to bake-out or initializing the gas purification 

system. Cycle G-SOV1 and G-SOV2 (to the RGA) to ensure that they are closed.

(2) Wrap all components downstream of G-HOR1 and G-HOR2 with heater tape to allow bake-out 

of the gas fill lines, valves, and mixture bottle. Strategically place thermocouples on the lines and components to monitor temperature. (Use the handheld thermocouple (TC) meter to read thermocouples.) Place at 

least one layer of aluminum foil over the heater tape to minimize thermal losses to the environment. (Note: 

The gas supply bottles and regulators are not included in this bake-out procedure as they are upstream 

of the purification system and the regulators cannot withstand the temperatures required for bake-out.)

(3) Evacuate the gas feed system using the evacuation port by opening G-HOV16 and 

G-HOV17. Then open valves G-HOV12, G-HOV15, G-HOV9, G-HOV8, G-HOV1, and G-HOV2 to 

rough down the complete feed system. (Note: Valves G-HOV13, G-HOV14, G-HOV4, and G-HOV6 

should remain closed to protect the microtorr and monotorr units. These valves should only open when 

system is under vacuum or charged with inert gas.)
53

 (4) Maintain pumping and achieve a vacuum condition of 10–3 torr or lower.

 (5) Open the monotorr and microtorr isolation valves G-HOV4, G-HOV6, G-HOV13, and 

G-HOV14 to remove any residual inert gas contained in the purifiers.

 (6) Maintain pumping and monitor the pressure (pressure should be 10–3 torr or lower); 

hold this condition for several hours.

 (7) During the hold period, use a He leak detector to test all fittings and valves for leaks. 

Set the detectors range to read a minimum of 1×10–9 std cc/s He.

 (8) Once leak check is completed and pressure has reached a stable low value, apply power to 

the heater tapes to increase the temperature of the gas feed system and all components to ≈250 oC. Hold 

this temperature for a minimum of 12 hr. System bake-out of 24 hr is desirable.

 (9) Monitor test chamber vacuum level; there should be an initial increase in pressure followed 

by a slow decay as the outgassing rate decreases.

(10) Turn off heater tapes and let system cool to room temperature; continue evacuating 

the system during cooling.

(11) Close the monotorr and microtorr isolation valves G-HOV4, G-HOV6, G-HOV13, and 

G-HOV14 to protect the purifiers. 

(12) Close evacuation port isolation valves G-HOV16 and G-HOV17.

(13) The gas feed system is now ready to be charged with required gases.

5.1.4.2 Test Chamber Bake-Out. The following are procedures for test chamber bake-out:

 (1) Verify that chamber isolation valves G-HOV10 and G-HOV11 are closed.

 (2) Wrap the test chamber with heater tapes. Strategically place thermocouples on test chamber 

to monitor temperature. (Use the handheld TC meter to read thermocouples.) Place at least one layer of 

aluminum foil over the heater tape to minimize thermal losses to the environment. Also place heater tape 

on the RGA sensing lines up to the RGA internal isolation valve. (Note: Allowable bake-out temperature 

at the RGA interface is lower than that used on the rest of the system.) 

 (3) Evacuate test chamber using roughing/turbopump combination to a pressure of 10–6 torr 

or lower; monitor pressure.

 (4) Open RGA isolation valves G-SOV1 and G-SOV2 such that the sensing line is evacuated 

up to the RGA internal isolation valve.
54

 (5) Maintain pumping with a vacuum condition of 10–6 torr or lower and hold for several hours.

 (6) During the hold period, use a He leak detector to test all fittings and valves for leaks. 

Set the detectors range to read a minimum of 1×10–9 std cc/s He.

 (7) Once leak check is completed and pressure has reached a stable low value, apply power 

to the heater tapes to increase the temperature of the test chamber and RGA sensing lines to ≈250 oC. Hold 

this temperature for a minimum of 12 hr. System bake-out of 24 hr is desirable.

 (8) Monitor test chamber vacuum level. There should be an initial increase in pressure 

followed by a slow decay as the outgassing rate decreases.

 (9) Turn off heater tapes and let system cool to room temperature; continue evacuating 

the system during cooling.

(10) Close the RGA isolation valves G-SOV1 and G-SOV2 to protect the RGA. 

(11) Close turbopump isolation valve and roughing valve locking up a vacuum inside the 

turbopump.

(12) The test chamber is now ready to be loaded with gases using multiple dilution cycles.

5.1.4.3 Charging the Gas Mixture Bottle. The procedure for charging the gas mixture bottle 

follows:

(1) Ensure that the gas fill lines are isolated from the recirculating gas purification system by closing the test chamber gas system isolation loading valve G-HOV9.

(2) Open gas mixture bottle isolation valve G-HOV8. 

(3) Open microtorr purifier isolation valves G-HOV4 and G-HOV6 to fill the test chamber. 

(4) Open Ar isolation valve G-HOV1. Slowly open Ar regulator G-HOR1 to fill the gas mixture 

bottle to the required partial pressure for the desired gas mixture:

• He-32%Ar: 32 psig Ar

• He-6%Ar: 6 psig Ar.

(5) Close Ar supply regulator and isolation valve G-HOR1 and G-HOV1.

(6) Open He isolation valve G-HOV2. Slowly open He regulator G-HOR2 to fill the gas mixture bottle to a total pressure of 100 psig, achieving He partial pressure of 68 psig and 94 psig for the 

32% and 6% Ar mixtures, respectively.

(7) Close He supply regulator and isolation valve G-HOR2 and G-HOV2, respectively.
55

(8) Close microtorr purifier isolation valves G-HOV4 and G-HOV6.

(9) The gas mixture bottle is now charged with an appropriate mixture ready for loading into 

the vacuum chamber. 

5.1.4.4 Gas Loading the Test Chamber. The following procedure is for gas loading the test 

chamber: 

(1) Open the test chamber isolation valve G-HOV10. 

(2) Open test chamber gas system isolation loading valve G-HOV9.

(3) Slowly open gas mixture bottle isolation valve G-HOV8 and fill the test chamber to a pressure 

of ≈76 torr; close G-HOV8 once the pressure is reached. 

(4) Close test chamber gas system isolation loading valve G-HOV9.

(5) Open RGA isolation solenoid valve G-SOV1 so that the test chamber gas mixture can be 

sampled.

(6) Evacuate test chamber to a pressure of 10–3 torr or lower and hold for 30 min.

(7) Repeat steps (2)–(6) until the RGA shows a maximum oxygen concentration on the order of 

0.3 ppb. This should be achieved in two fill cycles.

(8) Close test chamber isolation valves G-HOV10. 

(9) Test chamber system is ready for operation.

5.1.4.5 Initialize the Recirculating Gas Loop. Initializing the recirculating gas loop procedure 

follows:

(1) Verify that test chamber isolation valves G-HOV10 and G-HOV11 are closed. 

(2) Verify that monotorr isolation valves G-HOV13 and G-HOV14 are closed.

(3) Open RGA isolation solenoid valve G-SOV1 so that the test chamber gas mixture can be 

sampled.

(4) Open gas system bypass valves G-HOV12 and G-HOV15.

(5) Start the gas pump. (Both the test chamber and the monotorr have been isolated from the 

system.)

(6) Open monotorr isolation valves G-HOV13 and G-HOV14 to initiate flow through the monotorr with the test chamber still isolated from the system. Turn on power to the monotorr unit.
56

 (7) Close gas system bypass valve G-HOV15.

 (8) Open test chamber isolation valves G-HOV10 and G-HOV11 such that gas will be cycled 

both through the test chamber and the monotorr purifier. Maintain this state of operation throughout 

testing as needed.

 (9) Close gas system bypass valve G-HOV12.

(10) Periodically monitor test chamber atmosphere with RGA.

(11) Test chamber is ready to support test operations.

5.2 General Procurement Specifications

Primary hardware procurements necessary to complete the assembly and operation of the proposed gas handling system have been identified. Solicitations will be issued by MSFC using full and 

open competition. The final evaluation of vendor quotes will be based on the lowest cost technically 

acceptable approach. The current baseline requires two test chambers, each of which will make use 

of purified gas—the large chamber with three test sections containing 15 heat pipes and the smaller 

chamber containing a single heat pipe. These systems will share a common inert gas supply system 

and primary inlet purifier. However, two complete gas conditioning systems (ratios of He-32%Ar 

and He-6%Ar) and two complete purifying circulation systems attached to each test chamber will be 

required. The primary components within test systems include the following: 

(1) Microtorr purifier units (marketed by SAES Inc.) to provide single-pass control to the common gas supply hardware. These are commercial off-the-shelf items with a delivery of 4 to 6 wk.

(2) Monotorr purifier units to provide the primary conditioning over the life of the test evaluation. 

These units are commercial off-the-shelf items with a delivery of 4 to 6 wk.

(3) Recirculation pump (sealed bellows-type unit or equivalent with low permeability and particulate generation) to circulate the inert gas mixture through the monotorr purifier. These units will in all 

probability be a modified version of commercial off-the-shelf items. Delivery is estimated to range from 

8 to 16 wk. To conserve funds, the lowest cost technically acceptable unit will be pursued.

(4) Vacuum pump system to condition and recycle the test chamber and gas handling system. Two 

turbopump units will be used, a 300-L/s pump for the large chamber and a 70-L/s pump for the smaller 

chamber. A single roughing unit should be sufficient with a 600-L/min rating. These units are off-the-shelf 

items with a delivery of 4 to 8 wk.

(5) A variety of gas system plumbing components such as isolation valves, relief valves, tubing, 

flow meters, fittings, instrumentation, and structural materials are required. All of these items are commercial off-the-shelf available from a variety of vendors with typical deliver of 4 wk or less.
57

(6) Part fabrication and assembly at machine shops, as required to complete system.

To assist in setting up the gas system, there are a number of items that can be borrowed from 

in-house inventory and other projects while these systems are being built and tested. Appendix K 

provides a more detailed listing of primary components selected for use in the gas purification system.
58

6. INSTRUMENTATION

The planned operations for the accelerated life test heat pipe experiment will require roundthe-clock testing for intervals of up to 6 mo with the full duration expected to reach 3 yr. To maintain 

the test system throughout this period, a robust data and control system must be in place. This system 

will consist of a combination of computers and distributed real-time controllers (RTCs) connected via 

a local area network. The computer(s) will record the test data and provide the user with an interface 

for monitoring and controlling operations. The distributed RTCs will provide independent monitoring 

of all key instrumentation and contain uploaded programming for normal operations, redline conditions, 

and shutdown protocols. The RTC provides the first line of defense in maintaining the overall health 

of the system. In the event of a computer error/crash or localized power outage, the RTCs will also 

safely shut down and/or place components in a safe mode, using stand-alone internal programming. 

The selection of instrumentation type and quantity is critical. The most durable types of instruments 

must be selected to prevent inadvertent test cuts due to an instrument malfunction. This necessitates 

components that are noncontact type and have few moving parts. Having the minimum amount of 

instrumentation needed to achieve the test objectives is also important. Unnecessary instrumentation 

results in larger, unwieldy data files and increases the risk of a false cut should one of the “extra” 

instruments fail. Additional instrumentation should only be added for monitoring purposes and should 

not be used to generate control interrupts. Finally, all instrumentation must be readily accessible, except 

for items located inside the test chambers, such that they are easy to repair or replace in the event 

of a malfunction. 

6.1 Data Acquisition and Control Methodology

The test setup is divided into a number of subsystems, each with its own control requirements 

and instrumentation for monitoring and control purposes. To achieve test objectives, the integrated test 

system must operate continuously without regular operator observation. This implies automated routines 

with criteria, such as limits and operating bands, governing the establishment of warning, shutdown, 

and cut conditions. The method in which test warnings and cuts are implemented is extremely important, 

especially with regard to startup transients. They must be implemented such that artificial cut conditions, which prevent the hardware from reaching its steady-state requirements, are not introduced. A series 

of checkout tests will be necessary to determine the best methods for implementing both warning and cut 

protocol. At present, a total of four primary subsystems have been identified:

(1) Radio frequency power supplies and power distribution network—This subsystem provides 

both monitoring and control functions, including RF power system current, voltage, power, frequency, and 

coolant operating temperature. Each RF power supply is factory equipped with internal systems to protect 

against over-temperature or current conditions that will shut the unit down immediately; however, these 

internal controls should not be relied upon for the controlled termination of a test. Several primary redline 

cuts based on supply output power and inductive coil water-coolant temperature and flow are envisioned. 

The typical warning band for RF system power would be ±6% with a cut condition set to ±10%. Typical coolant flow and temperature warning bands would both be set to ±10% with a cut at ±15%. If a flow 
59

switch rather than a flow meter is used on the water-cooling circuit, a loss of flow indication will be used 

as the cut condition. (The use of flow switches must be examined in more detail as past experience with 

these instruments has not been favorable.) In the event of a cut condition, the programmable logic controller (PLC)/computer will shut down the RF power supplies. The water coolant systems could be shut down 

a short time later if necessary, allowing sufficient time to cool hot components. 

(2) Test chamber environment gas conditioning circuit—This subsystem only provides monitoring functions of line temperatures, flow rate, and test chamber pressure. The primary redline cut currently 

envisioned is based on deviation in test chamber pressure (high/low), which would signal a potential 

leak—atmospheric, coolant, or heat pipe. The typical warning band would be set to ±8% with a redline cut 

set to ±12% of the nominal test set condition. In addition, this system also includes monitoring and control 

of the RGA, including cycling of its solenoid isolation valves, and providing data to the data acquisition 

system regarding test chamber gas species partial pressures. In the event of a cut condition, the RTC/computer system would command the RF power supplies to shut down immediately. Termination of coolant 

flow could also be commanded shortly after RF power shutdown once sufficient heat has been removed 

from components to prevent hardware damage. 

(3) Heat pipe calorimeter water circulation circuit—This subsystem only provides monitoring 

functions, including calorimeter temperatures, flows, and overall system pressure. Three primary redline 

cuts are envisioned. The first two cuts are based on variations from the nominal operating flow and exit 

temperature for each heat pipe. The typical warning band would be set to ±10% while that for a cut would 

be set at ±15% of the nominal test set conditions. The third cut is based on bulk facility coolant flow and is 

indicated by a flow switch (or flow meter). Detecting the loss of facility coolant flow allows for an orderly 

shutdown of all RF power supplies since there will be sufficient thermal capacity within the calorimeter 

circulation coolant loop to remove any excess heat, thus avoiding potential damage to test hardware or 

ancillary systems. 

(4) Heat pipe temperature sensing—This subsystem provides monitoring only of the heat pipe 

evaporator temperature in the small gap between the RF inductive coil and the calorimeter. There is one 

proposed primary redline cut based on a high/low temperature indication from the heat pipe surface. The 

typical temperature warning band would be set to ±6% and the redline cut to ±10% of the nominal test conditions. During initial test evaluation, periodic observation of the temperature measurement trends will be 

required to determine the level of temperature shifting due to potential (slow) changes in material surface 

emittance, impacting the radiation component of overall heat transfer. In the event of a cut condition, the 

PLC/computer will command the RF supplies to shut down. Water cooling will continue as necessary to 

remove residual heat from the test hardware and support equipment.

As evidenced by this summary, there are a large number of individual responses that can be used 

to indicate both warning and shutdown conditions. In many cases, it may be important to implement 

a voting structure or combination of these responses to better determine the overall magnitude of a warning. A properly constructed interpretation routine could possibly differentiate between instrumentation 

malfunctions and actual heat pipe or support subsystem failure, which will aid in troubleshooting. For 

example, if while monitoring the combined heat pipe temperature, calorimeter coolant flow, and calorimeter outlet temperature, one of the instruments fails while the other two continue to track per the nominal 

test conditions, then instead of performing a cut, a warning mode could be entered until the malfunctioning instrument can be replaced.
60

Instrumentation within each of these subsystems is tied into an RTC that provides data to the host 

computer via a local network. In addition, a visual alarm indicator will be set up to continuously status the 

current machine condition. This alarm will consist of a green, amber, and red light arrangement visible 

throughout the test laboratory. The normal status indicator is green and remains lit when all subsystems are 

operating within allowable limits. The amber indicator signals a warning that one or more of the monitoring parameters are nearing a cut status or that a subsystem has suffered a recoverable fault. (It is recommended that the user examine the cause of the warning.) A shutdown condition is indicated by a red light, 

signaling that the test has been terminated due to a redline violation or an irrecoverable subsystem fault. 

Each of the data system components will be powered using an uninterruptible power supply that will provide at least 10 to 15 min of operation in the event of a full-facility power loss. This will allow sufficient 

time to monitor the ambient cooling of all components and provide final commands to be issued by all 

RTCs and test components so that if power returns, the system will be placed in a safe shutdown mode. 

The general layout of the subsystems is illustrated in figure 28. 

Radio Frequency

Power System

Heat Pipe

Temperature

System

Calorimeter WaterCooling System

Gas Conditioning

System

Distributed Input/Output

FieldPoint System

Real-Time Controller

Operator

Console

(Computer)

Figure 28. General data acquisition and control system subsystems.

The hardware that will be employed for both data acquisition and control includes a National 

Instruments (NI), FieldPoint (FP) modular distributed I/O system (serves as the RTC), and an operator 

console running Windows XP professional. The FP units, which are real-time (RT) controllers, acquire 

data from instrumentation distributed within the subsystems and provide the capability to impose RT 

decisions and actions (related to limit criteria) on hardware components. The operator console will host NI 

LabVIEW© applications that store acquired data and provide a graphical user interface for operating and 

monitoring the system. After acquiring data, the FP units will distribute the data in real time to the operator 

console via a local area network. The RT controller will monitor the data. In the event of an out-of-limit 

condition, the RTC will execute preprogrammed instructions to shut down the RF power system and safe 

the system. The outlined control system will also be used to ensure that the heat pipe and support hardware 

are brought up to operating temperature in a controlled manner and maintained at their steady-state conditions. Continuous monitoring will be provided to verify overall system health.
61

6.2 Instrumentation Components and Procurement Specifications

6.2.1 Water-Cooling Subsystem

The RT application will monitor the water-cooling subsystem flow rates and temperatures for outof-limit conditions. In the event of coolant circulation failure or an over-temperature condition, the RF 

power to all or to the appropriate heat pipes will be interrupted. The specific flow rate and calorimeter 

temperature value limits shall be determined based on final pretest calculations and checkout test data 

for the assembled system. Depending on the final test chamber configuration, if an out-of-limit condition 

occurs in a limited portion of the system, it should be possible to selectively shut down the RF heating 

to the appropriate subset of heat pipes. In the event of a loss of coolant flow to the primary calorimeter 

recirculation system, a loss of flow in the facility supply, or a coolant over-temperature condition, all RF 

power supplies will be shut down. Appendix L provides a partial listing of the primary instrumentation 

components. This equipment is typically off-the-shelf with a 2- to 4-wk delivery.

As a baseline, each of the 16 calorimeter outlets will be equipped with a turbine flow meter (like 

the model FTB9511 from Omega Engineering or the MF150-CB-PH precision turbine from Sponsler) 

with a range of 0.1 to 2 gpm and a type K thermocouple in close proximity (to assess fluid properties 

for mass flow calculations). Alternatives to the turbine-type unit are a paddlewheel-type meter, such 

as the model 04004SN2 by Proteus Industries, and a thermal mass flow meter that has no moving parts, 

like the Weber model 4411.3 meter. The paddlewheel meter has the added benefit of also providing temperature and pressure measurements without the need for additional signal conditioning. All meter types 

shall be compared with a final decision based on a combination of cost, accuracy, and long-duration 

dependability/stability. Typical vendor availability is 4 to 8 wk.

To obtain the most accurate measure of the heat removed by the calorimeter, as determined 

from the temperature rise across the calorimeter, a thermocouple will be located inside the test chamber as close to the calorimeter output as feasible. This thermocouple will be attached to the surface of 

the copper calorimeter feed tube. As a backup, an additional thermocouple probe will be inserted, using 

a compression fitting, into the calorimeter output line after it exits the chamber, minimizing the potential 

for water leakage in the test chamber. Thermistors will be considered as replacement to thermocouples on 

the calorimeter to improve accuracy. 

The calorimeter recirculation flow circuit—upstream of the test chamber calorimeter inlet manifolds—will be equipped with a larger turbine-type flow meter, such as the Omega model FTB-905 

or Sponsler SP3/4-CB-PHL, having a linear range of 2.5 to 29 gpm and a local type K thermocouple. 

A thermal mass flow meter is not practical for this location due to the high flow rate. The facility water 

coolant inlet to the calorimeter recirculation subsystem heat exchanger will be equipped with a flow meter 

and both the inlet and outlet lines will be equipped with type K thermocouples. The cooling water inlet 

and outlet to the heat exchanger will also be equipped with type K thermocouples. The facility flow meter 

may be exchanged for a flow switch. However, inlet and outlet thermocouples will still be present to assess 

whether or not the facility system is providing both sufficient flow and cooling.
62

6.2.2 Gas Conditioning Subsystem

Purified inert gas is circulated throughout all test chambers during test processes and is periodically sampled by an RGA. The flow rate, temperature, and pressure of this gas will be monitored by the 

RT control system to confirm operation. In the event that the RT application monitors an out-of-limits 

flow condition, the RGA-measured partial pressures will be used to determine if the RF power subsystem 

should be interrupted or if the test be allowed to continue. It is anticipated that over long periods of operation, less scrubbing of the test chamber atmosphere will be required, such that a 100% duty cycle on the 

gas conditioning system may not be required. 

The RGA unit will be monitored by the RT controller used to automatically control the remote 

operated solenoid valves to sample the test chamber gas mixture at a user-specified rate. (The sampling 

rate is expected to be set at ≈6-hr intervals.) In the event that the gas composition is found to be out 

of limits, manual sampling can be performed to verify partial pressures and the RF power supplies 

can be interrupted as necessary. The initial warning will be set to 1 ppb oxygen with a high cut estimate set to 3 ppb oxygen. (The value of 3 ppb corresponds to a vacuum chamber pressure of 10–5 torr 

when evacuated from atmospheric conditions.) The RGA is equipped with a digital control interface (RS232 or RS-485) that allows for remote operation. Typically, the RT controller will evacuate the gas line 

connecting the RGA to the test chambers to clear it, then open one of the isolation valves (G-SOV1 or 

G-SOV2) to capture a gas sample that is analyzed for warning or cut limits. The isolation valve is then 

closed. This routine is repeated at the user-specified intervals for each test chamber. The isolation valves 

will be bellows-style valves with pneumatic actuators and solenoid controllers. These are off-the-shelf 

items. Also, there will be additional cost to set up the plumbing (hardware components), structural mounting, and electronic control interface between the RGA and RTC.

A flow meter is specified for the gas conditioning system since it provides an indication as 

to how well the system is circulating the gases; a flow switch would provide only binary output. This 

makes it easier to troubleshoot possible issues related to insufficient purification. The use of a flow 

switch is also questionable at the low system operating pressures. Measuring the mass flow in the gas 

circulation system will be difficult because of the low gas pressure (50–100 torr) and the potential 

for a variable mixture of He/Ar (adjusted to obtain the desired heat pipe heat flux). There are two types 

of flow meters that may be used—a thermal mass flow meter, such as the Fox Thermal Instruments 

Model 10A, or a laminar volumetric flow meter. The mass flow meter uses a thermal sensor to measure 

mass flow directly and, unlike the volumetric flow meter, is not affected by changes in pressure and 

temperature. With a volumetric flow meter, the pressure and temperature, along with the appropriate gas 

properties, must be used to compute the mass flow rate. Consequently, a thermal mass flow sensor can 

replace a volumetric flow meter and its associated pressure transducer. In addition, a thermal mass flow 

meter provides an accurate and repeatable measurement over its entire range of operation. Volumetric 

flow meters cannot provide this wide range of measurement and are especially inaccurate for low flow 

conditions. A thermal mass flow meter has no moving parts and is inherently more reliable than a volumetric flow meter. In addition, a turbine flow meter is not a good fit due to its low accuracy at low flow 

(zero to 150 slpm) conditions. Therefore, the thermal mass flow meter appears to be the best overall fit 

for the specific conditions within the gas system. A single mass flow meter, with appropriate calibration, 

has a delivery time of 4 to 8 wk. Specifics concerning the operation and possible calibration techniques 

for both the mass flow and laminar volumetric flow meters (issues with the He/Ar mixture) are included 

in appendix M.
63

6.2.3 Radio Frequency Heating System

The RF heating system will consist of four separate power supplies, one for each heat pipe cluster. Each of the power supplies will have three zero to 10-V analog inputs to control the frequency, current, and voltage, and four zero to 10-V analog outputs will be used to report the measured frequency, 

current, voltage, and power. The RT application will monitor the system for out-of-limit conditions. 

In the event of a limit violation, the RF power to the affected heat pipe cluster(s) will be interrupted. 

The RF supplies will be equipped with a commercial water-cooling station that will cool both the supplies 

and the RF inductive coils. This unit will be instrumented from the factory with flow indicators or meters 

and thermocouple probes that can be tied into the RT application and tested for both warning and cut conditions. To piece this system together, a variety of cabling, electronics, and support components will be 

required. These are typically off the shelf with 2- to 4-wk delivery.

6.2.4 Heat Pipe Temperature Measurement

A key parameter in assessing a heat pipe’s performance is the temperature at the evaporator exit. 

Two types of instruments can be used to measure this temperature, and a trade-off assessment must be 

made to select the most suitable type. First, a traditional type C thermocouple could be attached to the heat 

pipe surface. Alternatively, a two-band optical pyrometer can provide the measurement using a noncontact 

technique. A number of the comparisons are summarized in table 14.

Table 14. Thermocouple to pyrometer comparison.

Two-Band Pyrometer Type C Thermocouples

Expensive hardware Inexpensive hardware

Mounted outside of the test chamber; no contact with heat pipe Must be directly attached to the surface of the heat pipe by bonding or spot welding

No potential for debonding or contamination of heat pipe alloy If bonded, frequent debonding occurs. For long-term testing, the nickel foil used in 

bonding will diffuse into the heat pipe alloy

No potential for pitting the heat pipe surface If spot welded, pitting of heat pipe surface can occur and potentially embed or trap 

impurities

Eases operations involving loading and unloading of heat pipes Over the duration of the program there will be many bonding/spot welding cycles 

covering nondestructive evaluation and thermocouple repair

No potential for localized cooling Leads act as fins, locally cooling at the measurement location

Minimizes potential for degradation due to high temperature; 

two-band reduces affect of surface emissivity changes

Long-term degradation of thermocouple wires at high temperature introduces 

unknowns

Easy to repair; unit is positioned external to the test chamber Difficult to access; located inside the test chamber, requiring backup thermocouples 

to be attached

Low maintenance cost with minimal down time and manpower 

usage for repair

High maintenance cost with large down time and manpower usage for repair

Based on the summary provided in table 14, it is readily evident that the use of two-band optical 

pyrometers for all heat pipe measurements will significantly reduce the overall setup and maintenance 

effort when compared to the use of thermocouples inside the test chamber system. This, coupled with 

overall reliability and the elimination of potential contamination or damage to the heat pipe, provides justification for the selection of the two-band pyrometer for use in the baseline system for the planned testing.
64

Two optical pyrometer units have been identified—the MI-SQ5 series from Mikron and the Modline 5R series from Ircon. Details for each these units are listed in appendix N. The two-band pyrometer makes high-temperature measurements by obtaining the ratio of radiation intensities at two adjacent 

wavelengths rather than using the absolute intensity, as with the single-band pyrometer. The vendor 

reports that this technique results in a temperature measurement that somewhat reduces the dependence 

on emissivity, is unaffected by dirty viewing windows and contaminants in the field of view, is independent of target size, as long as a minimum percentage of the target falls within the sighting reticule, and 

is unaffected by a moving target within the field of view. Both models support an RS-485 digital interface that will allow the RT controller to communicate with multiple sensors over a single RS-485 connection. The distance from the outside edge of the glass to the heat pipe surface is ≈12 in. However, this 

may decrease to 11 in or less if the RF inductive coils are spaced out to reduce inductive coupling. This 

distance is within factory-available focal lengths and will accommodate a spot diameter of <0.24 in. 

A standard temperature range of 1,000 to 1,500 K will easily cover all operations. To verify the operation and ease of use, checkout testing is planned using loaner units from the factory. Current vendor 

information indicates, depending on the options selected, 17 optical pyrometers will be required. Availability is typically 6 to 8 wk. Additional materials/fabrication of support brackets, adjustable mechanical 

positioners, and cabling to set up and connect all the pyrometers to the test chambers and data system will 

also be needed.

In addition to the pyrometers, a supplemental luminosity measurement is currently being investigated to monitor each cluster of heat pipes for potential hot spots on the heat pipe evaporator that might 

not be visible to the two-band pyrometer. The proposed technique will make use of a photodiode in 

a view port at the evaporator end of the heat pipe cluster. From this vantage point, the diode could 

pick up any significant increase in brightness—produced by a dryout—through the 0.25-in-gap width 

between the RF coil and heat pipe evaporator. When the system is at its normal operating conditions, 

the average sensed luminosity (brightness) will be recorded and set as the baseline. Any increase above 

this brightness level would result in a warning or cut condition with typical values of 5% and 10%, respectively. If any cut limit is exceeded, the RT controller will shut down the RF power supply.

Within the test chamber, a series of low-temperature surface measurements will be taken at nonrefractory metal locations using type K thermocouples spot welded to the surface. These locations will 

include heat pipe supports and the calorimeter flow outlet, totaling ≈32 thermocouples inside the test 

chambers.

6.2.5 Test Chamber Vacuum Gauging

To monitor test chamber pressure conditions during all operational phases, including bake-out, gas 

conditioning/loading, and normal test procedures, a vacuum pressure system will be required. This system 

must have sufficient instrumentation to measure a pressure range from 1 atm to <10–6 torr. Three types 

of gauges are needed to cover this range—a capacitance manometer (1,000 to 1 torr), a convection Pirani 

gauge (1 to 10–3 torr), and a cold cathode gauge (10–3 to 10–8 torr). The preferred solution is a multigauge 

controller, like the KJLC multigauge (marketed by the Kurt J. Lesker Company). This instrument has five 

slots for sensor boards and one slot for an RS-232 or RS-485 serial interface board. Appendix O provides 

a listing of the component specifications. Vendor estimated availability is 2 to 4 wk.
65

6.2.6 FieldPoint-Distributed Inlet/Outlet System and Real-Time Controller

All data acquisition and control will be performed via an NI FP modular distributed I/O system. 

The system will include modules for reading and writing analog current and voltage signals, modules 

for reading thermocouples, and modules for reading and writing digital signals. The FP system will also 

include an RT controller, such as the FP-2010 marketed by NI, for performing automated control and 

for monitoring measurements used in comparison with out-of-limit conditions. The RT application will 

be developed with LabVIEW and will be downloaded to the controller. An RT controller was chosen 

to handle this functionality because it is more reliable and deterministic than a computer running a Windows OS. The RT controller’s operating system is not burdened with the user interface, data recording, 

and overhead associated with a Windows desktop OS. Also, this controller is compatible with existing 

instrumentation and development tools already being used within the laboratory. The RTC will be used 

to acquire all measurements (flow rates, temperatures, and pressures), perform closed-loop control of 

the RF power supplies, and monitor measurements for limit (warning and cut) violations. Appendix P 

provides a listing of the outlined components and specifications. The current vendor estimated availability 

is 2 to 6 wk.

6.3 Data Quality

To maintain the highest quality test data, all sensor and data processing equipment must be calibrated at regular intervals. Data acquisition equipment and sensors will be received with standard factory 

calibration and, for an additional cost, can be delivered with a National Institute of Standards and Technology (NIST) traceable certification. All manufacturers recommend that their units be calibrated periodically, typically at yearly intervals. Some units, like thermocouples, cannot be recalibrated; they are simply 

replaced when they are out of calibration. There are NIST-certified devices available that can be used to 

field check their calibration. There are several options available to satisfy the requirement for periodic 

calibration, each carrying a different level of expense and impact to the hardware systems: 

(1) The first option would require sending all units back to the factory or to a qualified standards 

lab. The turnaround time can range from 1 to 4 wk, depending on the company and work load. If testing is to resume during recalibration intervals, it will be necessary to purchase spare units and to send 

the units out only a few at a time. In this scenario, calibration would be a continuous process requiring 

a large number of components, rendering it a logistical nightmare. However, if all instrumentation is 

calibrated at the same time, as when the system is down for heat pipe nondestructive evaluation (NDE), 

this could streamline the process. Procurement for standards lab service will have to be initiated several 

months in advance to ensure that there will be no delays in calibration once the test hardware is opened 

for NDE. 

(2) A second option is to have the instrumentation calibrated in the field. It may not be possible to 

calibrate some sensors, e.g., flow meters, in the field. However, it should be possible to field calibrate the 

bulk of the instrumentation. Most of the cost would be to cover the service company’s technician travel 

and onsite time to perform calibrations. This approach may offer considerable cost savings relative to 

sending everything out for calibration, and will certainly reduce the down time. It will still be necessary 

to initiate the procurement for this service well in advance of its needed date. 
66

Another option to reduce cost is to periodically check the calibration in-house and to send out 

instruments for recalibrating only when they are found to be out of calibration. This may not be possible for all types of sensors but should be possible for most types of data acquisition and signal conditioning equipment. The in-house calibration check could be performed using a portable calibrator, like 

the Model X88 from Ronan. This instrument is designed to calibrate and measure instruments, devices, 

and systems utilizing current, voltage, ohms, frequency, and thermocouple inputs or outputs. Ronan 

recommends that the Model X88 be verified at 3- to 6-mo intervals using precision standards traceable to NIST. Since most instrumentation typically remains within calibration longer than the factory 

recommendation, this option may be the most cost effective. The downside to this approach is that NISTtraceable certifications will not remain in effect for the entire test, and instruments found to be out of calibration at critical times could impact testing. This option is the most desirable choice for maintaining the 

calibration of the FP I/O modules. These modules can be swapped out with a spare and checked without 

impacting the test. Modules found to be out of calibration can be replaced entirely and, if cost effective, 

sent out for calibration. 
67

7. SUMMARY

General descriptions of test subsystems to support the hardware evaluation of Mo-44.5%Re 

alloy/Na heat pipes have been identified. This includes both limited performance testing to determine heat 

pipe operational envelopes followed by round-the-clock testing of all 16 units. As identified, each of these 

subsystems is laid out to meet the requirement for a 3-yr period of continual operation. Initial outlines for 

hardware operating procedures have been identified where necessary and will be used during checkout 

testing of hardware during fabrication and assembly. Key subsystems include the following:

• Radio frequency heating of the 16 modules is very practical, making use of 30- to 50-kHz supplies 

with low voltage output (<200 V) to minimize the possibility of voltage breakdown. The inductive 

coils will be equipped with flux concentrators to minimize mutual inductance and stray heating, providing both a more uniformly heated area and less overall power consumption. Expected electrical 

to thermal efficiency is 50% to 70%.

• Coupling of the heat pipe condenser to a water calorimeter is based on the use of a static gas gap. The 

atmospheric concentration of oxygen at 0.3 ppb, comparable to that of a 10–6 torr vacuum, is required. 

Initial assessment shows that a gas recirculation circuit using a vacuum-type diaphragm pump and 

a hot getter purifier operating at a pressure of 76 torr can meet this goal.

• Separate water-cooling circulation subsystems will be procured to support operation of the RF power/

inductive coils and the heat pipe calorimeter assemblies. This system will have to remove an estimated 

85 kW during normal operation. 

The requirement for round-the-clock testing places a heavy burden on both the data acquisition 

and control subsystems. The high power levels needed for the complete setup further complicates this 

burden. Continuous monitoring of the heat pipes and each subsystem is provided to maintain up-to-date 

indication of the overall system health. The selected approach is to use a distributed system of computers 

and RT controllers connected through a local network. This approach provides the best overall redundancy, allowing the RT controllers to rapidly respond to both warning and redline issues with or without 

computer involvement.
68

APPENDIX A—ENVIRONMENTAL TEST CHAMBER ENGINEERING DRAWINGS

Figure 29 shows preliminary engineering drawings outlining the large environmental test chamber 

assembly with support stands. A total of three units are required for the proposed testing. The shell and 

support structure are shown in detail in figures 30 and 31, respectively.

Figure 29. Test chamber assembly (three segments integrated) (Dwg. No. 90M11906).
69

Figure 30. Detail test chamber shell (Dwg. No. 90M11906).
70

Figure 31. Detail test chamber support structure (Dwg. No. 90M11906).
71

APPENDIX B—HEAT PIPE CLUSTER SUPPORT ENGINEERING DRAWINGS

Preliminary engineering drawings outlining the heat pipe and calorimeter cluster attachment support structure and individual mounting bracket are shown in figure 32. A total of three support structures 

and 16 mounting brackets are required for the proposed testing. Detailed drawings of the attachment support structure are shown in figure 33 and the heat pipe and calorimeter mounting bracket in figure 34.

Figure 32. Heat pipe and calorimeter cluster attachment support and bracket assembly 

(Dwg. No. 90M11911).
72

Figure 33. Attachment support structure details (Dwg. No. 90M11911).
73

Figure 34. Heat pipe and calorimeter mounting bracket detail (Dwg. No. 90M11911).
74

APPENDIX C—CALORIMETER ENGINEERING DRAWINGS

Figures 35 and 36 show the preliminary engineering drawings detailing the layout of the copper 

(or stainless steel) heat pipe calorimeter element. A total of 16 calorimeters will be required to support the 

proposed testing.

Figure 35. Heat pipe calorimeter assembly (Dwg. No. 90M11898).
75

Figure 36. Calorimeter design detail (Dwg. No. 90M11898).
76

APPENDIX D—ENVIRONMENTAL TEST CHAMBER PROCUREMENTS

Table 15 provides a partial list of general hardware required for fabrication of the environmental 

test chambers, summarizing the primary components. All structural components, attachment hardware, 

and chamber envelope material are stainless steel. 

Table 15. General hardware needed for fabrication of environmental test chambers.

Item Model No./Description Specifications

Quantity 

Needed

Large flange sets 

Nor-Cal Products Inc.

WS-27-2400M

WS-27-2400F

27-in-diameter male wire seal flange, 

 bored

3 each M 

3 each F

Large flange sets 

Nor-Cal Products Inc.

WS-27-000M

WS-27-000F

27-in-diameter male wire seal flange, 

 blank

3 each M 

3 each F

Large pipe section 

Nor-Cal Products Inc.

NA suitable for vacuum application 24-in diameter by 34-in length – surface 

 electropolished

3 each

Small chamber flanged 

 full nipple 

Kurt J. Lesker

2×WSFB14X000M 

2×WSFB14X1200M 

Custom-length barrel must be quoted

12-in tube section with wire seal flange 

 sets on ends

1 unit

Test chamber ports and 

 flanges and connecting 

 hardware—bolts 

Nor-Cal Products Inc.

Variety of half nipples with conflate-type 

 flanges, bolts/washer sets, copper/wire 

 seals, feedthroughs, etc.

All hardware items for 4- and 2-in half-

 nipple tube flanges to equip/connect 

 all chambers and support hardware

Various

Optical view ports TBD – based on pyrometer needs TBD depending on final pyrometer 

 selection and wavelength

16

Optical view ports TBD – based on general access TBD size for general viewing 7

Structural material Variety parts to fabricate chamber supports 

 and internal frames. Mechanical position-

 ers. Calorimeters/heat pipes. Refractory 

 materials to set up internal supports

Angle, plate and tube stock, stainless 

 steel and refractory metal. Position-

 ers for adjusting heat pipe locations

NA

Vacuum relief valves Set point 15 psia, with large orifice Prevent test chamber overpressure – 

 vacuum compatible

4
77

APPENDIX E—CALORIMETER COOLING SUBSYSTEM PROCUREMENTS

A partial summary of the primary components required in the setup of the heat pipe calorimeter 

water circulation subsystem is shown in table 16.

Table 16. General hardware list for assembly of the heat pipe water-cooling subsystem.

Item Model No. Specifications

Quantity 

Needed

Might pure ultraviolet water purifier MP36C 12 gpm per unit 2

DI cartridge TBD 1 to 2 gpm 2

Calorimeter flow control valve 

Alabama Fluid Systems

SS-6BW SS plug valve 

3/8-in connections 

Max temp: 120 °C

20

Flow control valve 

Alabama Fluid Systems

SS-68TS32 

SS-65TS16 

With actuators

SS ball valve 

1- to 2-in connections 

Max temp: 120 °C

4

Calorimeter Isolation valve 

Alabama Fluid Systems

SS-62TS6 SS ball valve 

3/8-in connections 

Max temp: 120 °C

20

Isolation valve 

Alabama Fluid Systems

SS-68TS16 SS ball valve 

1- and 2-in connections 

Max temp: 120 °C

6

Bypass and drain valves 

Alabama Fluid Systems

SS-63TS8 SS ball valve 

1/2-in connections 

Max temp: 120 °C

7

Tubing, piping, flanges, and other 

 components/hardware

NA 3/8-,1/2-, 1-, 2-, and 4-in with fittings, 

 stainless steel and copper vacuum, 

 heaters, etc.

NA

Pressure relief valves 

Alabama Fluid Systems

RL4S8 SS 1/2-in relief; set pressure 100 psig 

Max temp: 120 °C

5

Recirculation water system skid 

Sentry Equipment Inc.

CWIS-35 Complete system with pump, reservoir, 

 flow control, and instrumentation

1
78

APPENDIX F—RADIO FREQUENCY INDUCTIVE COIL THERMAL 

LOAD WORKSHEET

General assessment of the RF inductive coil thermal load due to radiation and gas conduction 

between the heat pipe evaporator and inner surface of the RF coil and the estimated total coolant flow 

requirement due to thermal and internal coil heating (I2) losses are shown in table 17.
79

Table 17. Heat transfer loss estimates between RF inductive coil and heat pipe surface.

NRPCT WI#2 2/16/09

Heat Transfer Loss Estimates for RF Inductive Coil to Heat Pipe Surface

user input field

Heat Pipe Temperature 1373 K

Inductive Coil Avg Temperature 325 K Voltage Breakdown Relationship

V = 0.0162(P*D)2 + 5.3143(P*D) + 403.24

Emissivity of copper coil 0.75 where P= pressure in torr and D= distance in cm

Emissivty of Mo-Re heat pipe 0.15 and D is 1/2 the difference in the heat pipe OD and the Inductor coil ID

Boltzmann Constant 5.67E-08

Length Section under inductor 3 inches 0.0762 m

Heat Pipe Outside Diameter (OD) 0.625 inches 0.015875 m

Heat Pipe Area Under Inductor 5.8875 inches^2 0.00379838 m^2

Test Chamber Gas Pressure

Helium Thermal Conductivity 0.3 W/m-K 76 torr

He-30%Ar He-5%Ar He

Gap Heat Pipe Gap Heat Pipe Area Ratio Effective Radiation Conduction Total Heat Break Down Break Down Break Down Gap Heat Pipe

to Coil Gap to Coil Gap Heat Pipe Emissivity Heat Transfer Heat Transfer Transfer Voltage Voltage Voltage to Coil Gap

inches m to Inductor ID W W W V V V cm

0.025 0.00064 0.926 0.143 109.38 1954.91 2064.29 168.69 127.21 191.69 0.064

0.050 0.00127 0.862 0.144 109.71 1013.69 1123.40 207.30 153.14 229.13 0.127

0.075 0.00191 0.806 0.144 110.01 699.41 809.42 244.39 177.93 266.14 0.191

0.100 0.00254 0.758 0.145 110.27 541.91 652.18 280.01 201.63 302.72 0.254

0.125 0.00318 0.714 0.145 110.50 447.14 557.64 314.20 224.29 338.86 0.318

0.150 0.00381 0.676 0.145 110.70 383.76 494.47 347.00 245.95 374.57 0.381

0.175 0.00445 0.641 0.145 110.89 338.33 449.22 378.46 266.67 409.85 0.445

0.200 0.00508 0.610 0.146 111.06 304.13 415.19 408.64 286.49 444.69 0.508

0.225 0.00572 0.581 0.146 111.21 277.42 388.63 437.58 305.45 479.10 0.572

0.250 0.00635 0.556 0.146 111.35 255.96 367.31 465.31 323.60 513.08 0.635

0.275 0.00699 0.532 0.146 111.48 238.33 349.81 491.90 340.99 546.63 0.699

0.300 0.00762 0.510 0.146 111.60 223.57 335.17 517.39 357.67 579.74 0.762

0.325 0.00826 0.490 0.146 111.71 211.03 322.73 541.82 373.68 612.41 0.826

0.350 0.00889 0.472 0.147 111.81 200.22 312.03 565.24 389.07 644.66 0.889

0.375 0.00953 0.455 0.147 111.90 190.82 302.72 587.71 403.89 676.47 0.953

0.400 0.01016 0.439 0.147 111.99 182.55 294.54 609.25 418.18 707.85 1.016

0.425 0.01080 0.424 0.147 112.07 175.22 287.29 629.93 431.98 738.79 1.080

0.450 0.01143 0.410 0.147 112.14 168.67 280.81 649.79 445.36 769.31 1.143

0.475 0.01207 0.397 0.147 112.22 162.78 275.00 668.87 458.35 799.39 1.207

0.500 0.01270 0.385 0.147 112.28 157.46 269.74 687.23 471.00 829.03 1.270

Coil Loss Heating as a function of Distance Water Coolant DT 50 K

Loss (W) = 722.66*gap^2 + 3896.9*gap + 3725 Water Coolant Cp 4180 J/kg-K

Water Density 989 kg/m3

5 Inductive Coil Assemblies (1 Cluster)

Inductive Coil Coil Heating Total Water Total Water Percentage Percentage

Gap Heat Pipe Gap Heat Pipe Loss (internal) From Heat Total Coil Heat Coolant Flow Coolant Flow Coolant Flow Coolant Flow

to Coil Gap to Coil Gap Heating Pipe Load Rate Rate Coil Internal Heat Transfer

inches cm W W W kg/sec gpm gpm gpm

0.025 0.064 3975.4 10321.4 14296.8 0.34 5.4 1.5 3.9

0.050 0.127 4231.6 5617.0 9848.6 0.24 3.7 1.6 2.1

0.075 0.191 4493.6 4047.1 8540.7 0.20 3.2 1.7 1.5

0.100 0.254 4761.4 3260.9 8022.3 0.19 3.0 1.8 1.2

0.125 0.318 5035.1 2788.2 7823.3 0.19 3.0 1.9 1.1

0.150 0.381 5314.6 2472.3 7787.0 0.19 2.9 2.0 0.9

0.175 0.445 5600.0 2246.1 7846.1 0.19 3.0 2.1 0.9

0.200 0.508 5891.1 2075.9 7967.1 0.19 3.0 2.2 0.8

0.225 0.572 6188.1 1943.2 8131.3 0.19 3.1 2.3 0.7

0.250 0.635 6490.9 1836.6 8327.5 0.20 3.2 2.5 0.7

0.275 0.699 6799.6 1749.0 8548.6 0.20 3.2 2.6 0.7

0.300 0.762 7114.0 1675.8 8789.9 0.21 3.3 2.7 0.6

0.325 0.826 7434.3 1613.7 9048.0 0.22 3.4 2.8 0.6

0.350 0.889 7760.5 1560.2 9320.6 0.22 3.5 2.9 0.6

0.375 0.953 8092.4 1513.6 9606.0 0.23 3.6 3.1 0.6

0.400 1.016 8430.2 1472.7 9902.9 0.24 3.7 3.2 0.6

0.425 1.080 8773.8 1436.4 10210.3 0.24 3.9 3.3 0.5

0.450 1.143 9123.3 1404.1 10527.3 0.25 4.0 3.5 0.5

0.475 1.207 9478.5 1375.0 10853.5 0.26 4.1 3.6 0.5

0.500 1.270 9839.6 1348.7 11188.3 0.27 4.2 3.7 0.5

1/ ( )(1/ 1)

( ) 4 4

+ −

− =

i i o o

i hp ind

rad A A

A T T Q ε εσ

ln( )

2 ( )

o i

c hp ind

cond d d

L k T T Q − = π

( ) p out in

heating

C T T

Q m − & =
80

APPENDIX G—MOLYBDENUM-RHENIUM AND SODIUM RESISTIVITY PROPERTIES

Resistivity data were used in assessing coupling between the RF inductive coils and the heat pipe 

envelope/internal sodium (table 18).23 The only data readily available for the Mo-Re alloy are for Mo50%-Re, shown in table 19.24 This variation is expected to introduce minimal uncertainty since the alloy’s 

electrical performance should be nearly identical to that of the Mo-44.5%-Re, which is used in the actual 

heat pipes. Electrical resistivity curves for Na and Mo-50%Re are shown in figure 37.

Table 18. Sodium resistivity.

Temperature

(K)

Resistivity

(μW-cm)

373 10

473 13

523 14.8

573 16.5

623 18

873 27

1,073 34

1,273 41

1,473 48

Table 19. Mo-50%Re resistivity.

Temperature

(K)

Resistivity

(μW-cm)

273 5

773 30

1,023 40

1,273 48

1,773 60
81

Temperature (K)

0 500 1,000 1,500 2,000

70

60

50

40

30

20

10

0

Na

Mo-50%Re

Resistivity (μΩ-cm)

Figure 37. Electrical resistivity curves for Na and Mo-50%Re.
82

APPENDIX H—RADIO FREQUENCY INDUCTIVE COIL ONE-DIMENSIONAL

SIMULATION RESULTS

Results generated using the ELTA software to produce one-dimensional results with the following 

input assumptions are shown in table 20:

Heat pipe temperature: 1,373 K

Heat pipe power condition: 3,000 W

RF coil/heat pipe heated length: 6.5 cm

Heat pipe outside diameter: 1.60 cm

Heat pipe inside diameter: 1.35 cm

Sodium layer thickness: 0.063 cm

RF inductive coil: One turn over heat pipe evaporator.

These results assume no heat pipe thermal loss except for that to the RF coil (gap dependent) as 

given by:

For 3.2-mm gap: 560 W

For 6.4-mm gap: 370 W

For 9.6-mm gap: 300 W.
83

Table 20. Radio frequency inductive coil one-dimensional simulation results using 

one- and five-coil calculations.

One-Coil Calculations

(Does Not Include Busswork)

Five-Coil Calculations

(Does Not Include Busswork)

Frequency 

(Hz)

Gap

(mm)

Electrical

Efficiency

(%)

Cal. Power

(W)

Pipe

Power

(W)

Total

Power

(W)

Coil 

Voltage 

(V)

Cal. Power

(W)

Pipe

Power

(W)

Total

Power

(W)

Coil 

Voltage 

(V)

3,000 3.2 44.3 3,000 3,557 8,029 2.3 15,000 17,785 40,147 12

10,000 3.2 74.7 3,000 3,557 4,762 2.5 15,000 17,785 23,809 13

20,000 3.2 78.7 3,000 3,557 4,520 3.1 15,000 17,785 22,598 16

30,000 3.2 77.9 3,000 3,557 4,566 3.9 15,000 17,785 22,831 20

50,000 3.2 75.9 3,000 3,557 4,686 5.6 15,000 17,785 23,432 28

100,000 3.2 73.9 3,000 3,557 4,813 9.5 15,000 17,785 24,066 48

500,000 3.2 75.6 3,000 3,557 4,705 27.8 15,000 17,785 23,525 139

3,000 6.4 37.4 3,000 3,367 9,003 3.6 15,000 16,835 45,013 18

10,000 6.4 68.7 3,000 3,367 4,901 4.2 15,000 16,835 24,505 21

20,000 6.4 73.0 3,000 3,367 4,612 5.7 15,000 16,835 23,062 29

30,000 6.4 72.1 3,000 3,367 4,670 7.5 15,000 16,835 23,350 38

50,000 6.4 69.8 3,000 3,367 4,824 11.3 15,000 16,835 24,119 57

100,000 6.4 67.5 3,000 3,367 4,988 19.5 15,000 16,835 24,941 98

500,000 6.4 69.3 3,000 3,367 4,859 60.5 15,000 16,835 24,293 303

3,000 9.6 31.9 3,000 3,302 10,351 5.3 15,000 16,510 51,755 27

10,000 9.6 63.2 3,000 3,302 5,225 6.3 15,000 16,510 26,123 32

20,000 9.6 67.8 3,000 3,302 4,870 8.9 15,000 16,510 24,351 45

30,000 9.6 67.0 3,000 3,302 4,928 12 15,000 16,510 24,642 60

50,000 9.6 64.1 3,000 3,302 5,151 18.4 15,000 16,510 25,757 92

100,000 9.6 61.5 3,000 3,302 5,369 32.3 15,000 16,510 26,846 162

500,000 9.6 63.5 3,000 3,302 5,200 101.2 15,000 16,510 26,000 506
84

APPENDIX I—RADIO FREQUENCY INDUCTIVE COIL TWO-DIMENSIONAL 

SIMULATION RESULTS

Results generated using the Flux 2D (marketed by CEDRAT Technologies) software to produce 

two-dimensional results with the following input assumptions are shown in table 21:

Heat pipe temperature: 1,373 K

Heat pipe power: 3,300 W

RF coil/tube length: 7.28 cm

Heat pipe outside diameter: 1.6 cm

Heat pipe inside diameter: 1.35 cm

Sodium layer thickness: 0.063 cm

RF inductive coil: Three turn over heat pipe evaporator.

Table 21. Radio frequency inductive coil two-dimensional simulation results using 

one- and five-coil calculations.

One-Coil Calculations

(Does Not Include Busswork)

Frequency

(Hz)

Gap

(mm)

Boundary

Type

Concentrator

Type

Electrical

Efficiency

(%)

Calorimeter

Losses

(W)

Pipe

Power

(W)

Total

Power

(W)

Coil

Current

(A)

Coil

Voltage

(V)

50,000

50,000

50,000

50,000

6.4

6.4

6.4

6.4

Infinity

Infinity

Close

Close

Fluxtrol 50

None

Fluxtrol 50

None

66.9

65.5

66.9

65.7

4.8

41.9

5

45.2

2,642

2,044

2,653

2166

3,949

3,122

3,964

3,298

1,300

1,300

1,300

1,300

29.3

25.2

29.4

25.9

One-Coil Recalculated for 3,300-W Pipe Power

(Does Not Include Busswork)

Frequency

(Hz)

Gap 

(mm)

Boundary

Type

Concentrator

Type

Electrical

Efficiency

(%)

Calorimeter

Power

(W)

Pipe

Power 

(W)

Total

Power 

(W)

Coil

Current 

(A)

Coil

Voltage 

(V)

50,000

50,000

50,000

50,000

6.4

6.4

6.4

6.4

Infinity

Infinity

Close

Close

Fluxtrol 50

None

Fluxtrol 50

None

66.9

65.5

66.9

65.7

6.0

67.6

6.2

68.9

3,300

3,300

3,300

3,300

4,933

5,040

4,931

5,025

1,453

1,652

1,450

1,605

33

32

33

32

Five-Coil Recalculated

(Does Not Include Busswork)

Frequency

(Hz)

Gap 

(mm)

Boundary

Type

Concentrator

Type

Electrical

Efficiency

(%)

Calorimeter

Power

(W)

Pipe

Power 

(W)

Total

Power 

(W)

Coil

Current 

(A)

Coil

Voltage 

(V)

50,000

50,000

50,000

50,000

6.4

6.4

6.4

6.4

Infinity

Infinity

Close

Close

Fluxtrol 50

None

Fluxtrol 50

None

66.9

65.5

66.9

65.7

30.0

338.2

31.1

344.3

16,500

16,500

16,500

16,500

24,663

25,202

24,654

25,123

1,453

1,652

1,450

1,605

164

160

164

160
85

APPENDIX J—RADIO FREQUENCY INDUCTIVE COIL ASSEMBLY DESIGN AND

FABRICATION WITH ASSOCIATED BUSSWORK SPECIFICATIONS

J.1 Integrated Financial Management Program Procurement

Description: Detailed design and precision manufacture of high-efficiency, long-life specialty RF inductive coil assemblies. These units should be ready for use and include feedthroughs and 

power feeds (busswork) to connect inductive coil assemblies to power supplies.

Material: Copper and other materials suitable for high-vacuum and low-pressure-purified inert atmosphere operation.

Items: • 2 × RF assemblies composed of five series inductor coils for equal power deposition.

• 1 × RF assembly composed of five series inductor coils for variable power deposition 

with a ratio of 1, 2, and 4.

• 1 × RF unit with a single inductor coil for constant power. 

• Busswork to connect each RF assembly to an RF power supply.

Delivery: 12 to 16 wk ARO

Suppliers:

 Vendor 1 Fluxtrol Manufacturing, Inc.

1388 Atlantic Boulevard

Auburn Boulevard

Auburn Hills, MI 48326–1572 USA

POC: Robert Goldstein 

Tel: 248–393–2000

 Vendor 2 Inductoheat, Inc.

32251 North Avis Drive

Madison Heights, MI 48071

POC: Michael R. Doerr

Tel: 724–327–5700

 Vendor 3 Ajax-Tocco Inc.

1506 Industrial Blvd.

Boaz, AL 35957

POC: Houston Lee

Sales: 256–840–2339

Source: Full and open competitive (not planning on using a sole source); final selection shall be 

based on lowest cost technically acceptable.
86

J.2 Radio Frequency Induction Coil and Network Specifications

It is required that four RF inductive coil assemblies be designed and fabricated for longduration (3 yr), continuous operation exposed to a high-temperature Mo-Re alloy tube (up to 1,400 K) 

in a vacuum or low-pressure-purified inert gas environment. The general pentagonal layout for three of 

the four assemblies was shown in figure 6. The fourth assembly is a single inductive coil. The vendor 

shall have sufficient RF inductive heating system design experience and two-dimensional electromagnetic 

modeling capabilities to thoroughly detail the performance of these assemblies for both nominal and offnominal operations. Prior to initiating fabrication, the performance estimates and two-dimensional modeling (with assumptions) shall be provided to MSFC for review and approval. Once approved, the vendor 

may proceed with manufacturing and test evaluation of the RF coil assemblies. The vendor shall state 

relevant experience, capability, and manufacturing specifications/guidelines in their response.

(1) Geometry—The general layout of a single heated tube element with characteristic dimensions is provided in figure 38. These inductive coil elements shall be arranged to form the pentagonal 

configuration with a distance between centers ranging from 3 to 4 in. The design objective shall be to 

set gap and turn sizing to maximize coil to tube coupling efficiency while minimizing heat loss and the 

potential for voltage breakdown.

RF Inductive 

Coil/Concentrator

Tube Section (One Position)

Behind RF Coil

Tube Outside 

Diameter

Tube to 

Inductor Gap Heat Length

Distance RF Coil Assembly to Calorimeter

Inductive Coil Inside Diameter

Tube Material: Mo-Re Alloy

Tube Wall Thickness: 0.05 in

Heated Length: 2.75 to 3 in

Tube Length Behind RF Coil: 1 to 1.5 in

RF Coil to Calorimeter Spacing: 0.2 to 0.25 in (Required Visible Path Cannot Be Blocked)

Tube Temperatures: 1,150 to 1,400 K

RF Coil ID to Tube OD Adjusted to Maximize Efficiency and Minimize Thermal Loss/Voltage Breakdown

Calorimeter

Tube

Figure 38. General dimensional layout of a single RF inductive coil position.

(2) Power—The RF inductive coil assemblies shall be designed to supply power in the manner specified in table 22 (refer to fig. 6). Radio frequency coil assemblies 1 and 2 have uniform power 

on all five elements in each series string. Radio frequency coil assembly 3 has three specified levels of 

power on the coils requiring a design assessment of the turn ratio. Radio frequency coil assembly 5 is a 

single inductive coil with no neighboring coils. During operation, the power variation between the coil

assemblies (1 and 2) should be minimal and not exceed 10% regardless of the actual power being dissipated; i.e., if the combined series coil assembly is set at 12,500 or 20,000 W, the dissipation per coil is 
87

Table 22. Required RF power induced in the part without thermal losses.

RF Coil 

Assembly

Position 1

(W)

Position 2

(W)

Position 3

(W)

Position 4

(W)

Position 5

(W)

1

2

3

4

3,000

3,000

1,000

5,000

3,000

3,000

2,000

3,000

3,000

2,000

3,000

3,000

4,000

3,000

3,000

4,000

constant and one-fifth of the total. Likewise, for coil assembly 3, during operation, there should be minimal variation between like positions not exceeding 10%, and the power ratio should be maintained regardless of the actual power level the assembly is operated at. 

(3) Open position performance—Periodically, it will be necessary to operate a cluster with 1, 

2, 3, or 4 of its positions empty (no tube located inside to heat). The RF coil assemblies and distribution network should be designed such that the positions that are filled still maintain the required power 

levels with tolerance, as specified in (2) Power above.

(4) Busswork—All busswork and test chamber feedthroughs shall be designed and manufactured 

for maximum efficiency. The power supplies shall be located approximately 6 to 12 ft away from the test 

chambers. 

(5) Efficiency—The RF inductive coil assemblies and busswork shall be designed to be as 

efficient as possible in coupling to the heated element and in making use of available power to satisfy 

the required applied power levels. The available frequency range for the RF power supplies for this 

application is zero to 50 kHz. 

(6) Uniformity of heated region—Each RF coil shall provide a uniform and symmetric heated 

zone under its full length with variations in power along its length targeting 5% to 10%. That means there 

should be no power tilting from front to back of the inductive coil and that it should be symmetric about 

the full circumference of the heated tube element.

(7) Mutual inductance and interference—The RF inductive coils shall make use of flux concentrators to minimize “cross talk” with neighboring coils within the cluster and structural elements that will 

adversely affect the uniformity of the applied power to the surface of the heated part. Additionally, the 

design and use of flux concentrators shall reduce coupling to adjacent components, such as calorimeters, 

to minimize the heating to tens of watts or less. Any flux concentrators shall be assembled of materials that 

comply with the vacuum requirement, bake-out requirement, and cooling, as necessary. 

(8) Materials and environment—The RF inductive coil assemblies shall be used in a test chamber with either a high vacuum or low-pressure-purified inert gas atmosphere. Contamination is a significant concern, therefore only materials suitable for high vacuum are acceptable. The manufacturing 

process must be orchestrated so as to eliminate the potential for trapped impurities or gas pockets that 

will continually outgas (create virtual leaks) during operation. Also, the final manufactured RF coil 

assemblies shall be capable of withstanding a thermal vacuum bake-out to a temperature of 150 to 

250 oC.
88

(9) Thermal environment—The RF coil shall be capable of continuous operation exposed to 

a hot tube surface (1,400 K) with duration of operation estimated at 3 yr. The final design shall dictate the 

final gap width between the heated surface and the inner diameter of the inductive coil. This design shall 

place the coil as far from the heated surface as possible to minimize thermal losses yet sufficiently close 

to achieve acceptable inductive coupling.

(10) Applied voltage—The final operating voltage for the complete RF inductive coil assembly 

should not exceed one-half that of the Paschen breakdown voltage for the geometry—the product of the 

minimum distance between the coil and any ground in centimeters with the ambient pressure in torr. 

(See fig. 39.) This requirement is to prevent a potential discharge that could damage either the heated 

test specimen or the RF coil. The test atmosphere shall be inert He at an absolute pressure of 10 to 

100 torr.

Pressure × Distance (torr-cm)

Breakdown Potential (V)

1 10 100

Helium

Argon

30% Argon

5% Argon

1,000

100

Figure 39. Direct current breakdown voltages in pure He, Ar, and mixtures (Paschen curves).

(11) Cooling—The RF inductive coil system and busswork shall make use of cooling water supplied at the RF power supply. (Water supply is not part of this procurement.) All mechanical connection 

on the RF power busswork and inductive coils must be made external to the test chamber. No leaks 

can be tolerated within the test chamber, therefore, all connections must be welded or brazed. The final 

assembly should be configured such that the temperature of the RF inductive coils should not exceed 

350 K during normal operations.

(12) Leak checking—No leaks can be tolerated within the test chamber; therefore, all connections 

must be welded or brazed. These assemblies shall be checked with a He mass spectrometer leak detector 

with a sensitivity capable of detecting leaks rates of 1×10–10 std-cc/s of He or lower.
89

(13) Cleaning—During final assembly, all RF inductive coil components and busswork that 

is located within the test chambers shall be maintained in a clean environment so as to not trap contaminates within the structure/materials that will outgas or create virtual leaks. These assemblies shall be 

cleaned (to degrease) as necessary using PF solvent and pure ethyl alcohol rinses and wiped down with 

lint-free material. The parts should be cleaned and packaged appropriately to protect them from contamination and damage due to handling. If the vendor has procedures to be used during the assembly process, 

provide them to MSFC for review and approval.

(14) Final acceptance and packaging—Documentation shall be generated regarding the design 

performance with results of two-dimensional analysis, hardware layout, hookup, and operation and provided to MSFC for final review/acceptance. The complete RF inductive coil assemblies, test chamber 

feedthroughs, and the power supply to chamber busswork and sensors shall be packaged in a manner 

to prevent damage or contamination and shipped to MSFC.
90

APPENDIX K—GAS PURIFICATION SUBSYSTEM PROCUREMENTS

Table 23 provides a summary of the primary hardware needs for buildup of the gas mixing and 

purification system for the heat pipe life tests. Some hardware can presently be borrowed from other projects to allow rapid buildup of the system; however, due to the extent of the heat pipe life tests (continuous 

testing for 3 yr), borrowed components will need to be replaced. Therefore, specifications and quotes are 

currently being collected for all items in the gas purification system, even if an item is available inhouse 

for temporary use.
91

Table 23. General hardware for gas purification subsystems (large/small chambers).

Item Model No. Specifications Quantity Needed

Monotorr phase II 3000 heated 

 getter purifier for rare gases 

SAES Pure Gas, Inc.

PS4-MT3-R-1 Max flow rate: 50 slpm 

Operating temp: 400 °C 

Max pressure: 100 psig

2

Monotorr ambient temperature 

 purifier for rare gases 

SAES Pure Gas, Inc.

PS11-MC200-R Max flow rate: 50 slpm 

Nominal flow rate: 5 slpm 

Max pressure: 150 psig

2

Monotorr ambient temperature 

 purifier for rare gases 

SAES Pure Gas, Inc.

PS11-MC500-R Max flow rate: 100 slpm 

Nominal flow rate: 12 slpm 

Max pressure: 150 psig

1

Turbopump unit 

Varian Vacuum Inc.

V301 Navigator 

Part No. 9698829

300-L/min unit with built-in controller 1

Turbopump unit 

Varian Vacuum Inc.

V70LP 

Part No. 9698829

70-L/min unit with built-in controller 1

Rough pump unit 

Alcatel Vacuum Tech.

ACP 40 multistage scroll 

Part No. V4SACA

600-L/min unit with long life 2

Swagelock stainless bellows 

 valve

SS-4H Stainless steel, 1⁄4-in tube fittings 

Max temp: 315 °C 

Max pressure: 1,000 psig

20

Swagelock stainless all weld 

 bellows valve

SS-8BW Stainless steel, 1⁄2-in tube fittings 20

Cyl-Tec, H-bottle* Model 220, DOT spec./

rating 3AA-2015

Steel, rated to 2,015 psi, 9-in OD

 ×51-in length, 43.4 L water 

Contact: Paul O’Neill, 630–844–8800

2

Pressure gauge, dial TBD 0–150 psig 3

Pressure gauge, dial TBD 0–1,000 torr 3

Regulator for K-bottle TBD 0–150 psi 3

Vacuum pressure relief valve TBD 120 psig 6

Vacuum pressure relief valve TBD 20 psia 8

Flow meter Mass flow type 0–20 slpm 3

Gas pump 

Senior Aerospace Metal 

 Bellows Division 

 (possible use)

Manuf No.: MB601DC Model MB-601 double containment, 

 high-pressure pump, aluminum 

 O-rings, 1⁄2-in female VCR connec-

 tions, max pressure rating: 100 psi, 

 max flow: 20 slpm, 110 VAC, 60-Hz 

 motor

1

Gas pump diaphragm type TBD 0–20 slpm operation at 10 to 760 torr 2

* Additional cleaning request will include an internal shot blast and treatment in 300 °F furnace. Cylinder will be valved immediately after 

 treatment and shipped with a low-pressure gas fill.
92

APPENDIX L—WATER-COOLING SUBSYSTEM INSTRUMENTATION

Table 24 provides a summary of the general instrumentation requirements (various types) to monitor the facility coolant flow, the recirculation subsystem flow, and the calorimeter temperatures. This does 

not include instrumentation for the RF power supply recirculation cooling subsystem. That equipment will 

be provided separately with that subsystem when it is procured. 

Table 24. General instrumentation options for water-cooling system.

Item Model No. Specification Qty

Weber Sensor Inc. 

Programmable flow-captor inline

 flow and temperature meter

Type 4411.3 Flow repeatability: <1% of FS 

Flow accuracy: <3% of FS 

Fluid temperature range: –20 to 100 °C 

Temperature repeatability: 1% 

Temperature accuracy: ±1 K 

Connections: tube 0.47-in OD with 0.039-in wall 

Flow range: 0 to 4 gpm 

Material: 316 stainless steel

17

Sponsler Lo-Flo 

Precision turbine flow meter

MF150-CB-PH-A-4X-V Repeatability: ±0.25% of reading 

Fluid temperature range: –267 to 204 °C 

Connections: 3⁄4-in MNPT 

Flow range: 0.1 to 2 gpm 

Max pressure: 5,000 psig 

Material: 304 stainless steel 

Rotor: 17-4 PH stainless steel 

Bearings: Cryogenic ball (other options available) 

Includes 20-point NIST certification for water

17

Proteus Industries 

Flow (paddle wheel), temperature 

 and pressure meter

04004SN2-TP Fluid temperature range: –40 to 140 °C 

Connections: 1⁄4-in FNPT 

Max pressure: 250 psi 

Output: User selected 0–5 VDC, 0–10 VDC, or 4–20 mA 

Flow range: 0.2 to 2.5 gpm 

Body: stainless steel 

Rotor: PPS 

O-ring: Silicone rubber 

Rotor shaft: 316 stainless steel 

Accuracy (flow, temp, & press): ±3% of FS (cal. points to ±1%) 

Linearity (flow, temp, & press): ±1% of FS 

Repeatability (flow, temp, & press): ±0.5% of FS

17

Omega Engineering 

Low flow turbine meter

FTB9511 Accuracy: ±1⁄2% of reading with linearization 

Repeatability: ±0.25% of reading 

Fluid temperature range: –268 to 232 °C 

Connections: 1⁄2-in MNPT std 

Flow range: 0.106 to 2.11 gpm 

Freq range: 40 to 1,050 Hz 

Max pressure: 5,000 psig 

Includes 20-point NIST certification 

 for water

17
93

Item Model No. Specification Qty

Omega Engineering 

Economical ball bearing liquid 

 turbine flow meter

FTB-905 Accuracy: ±1⁄2% of reading 

Repeatability: ±0.05% 

Body: 304 stainless steel 

Rotor: 17-4 pH SS 

Bearings: 440 °C stainless steel 

Fluid temperature range: –200 to 200 °C 

Connections: 3⁄4-in MNPT std 

Linear range: 2.5 to 29 gpm 

Nominal K-factor: 1,035 pulses per gallon (43.125 to 500.25 Hz) 

Max pressure: 5,000 psi NIST certificate supplied

3

Sponsler Precision 

Turbine flow meter

SP(3/4-in)-CB-PHL-A-4X Repeatability: ±0.5% 

Premium Repeatability: ±0.2% (over a specific range) 

Linearity: ±0.1% 

Premium linearity: ±0.2% (over a specific range) 

Fluid temperature range: –267 to 232 °C 

Connections: 3⁄4-in MNPT 

Flow range: 2.5 to 29 gpm 

Max pressure: Accommodates unlimited pressure depending 

 on the end fittings 

Material: 304 stainless steel 

Rotor: 17-4 PH stainless steel 

Bearings: Cryogenic ball (other options available) 

Includes 10-point NIST certification for water

3

Flow measurement remote 

 indicator/totalizer (option to 

 consider for signal conditioning)

IT400-DC-TRL-X Operating temperature: –30 to 75 °C 

Input voltage: 48 VDC

Signal input:

 Frequency: 0–3 kHz 

 Amplitude: 50 mVmin 

Analog output: 4–20 mA 

Analog output accuracy: 0.025% F/S @ 20 °C 

Digitial output: RS-232 optional (RS_485 not avail.) 

Digital output accuracy: ±0.01% reading (rate) or ±1 count (total)

19

Moore Industries 

PC-programmable frequency-

 to-DC transmitter with display 

 (option to consider for signal 

 conditioning)

FDY/PRG/4-20MA/ 

12-42DC/-CALIB [DN]

Operating temperature: –40 to 85 °C 

Input voltage: 12 to 42 VDC 

Signal input:

 Frequency: 0.005 Hz to 25 kHz 

 Amplitude: 50 mVmin 

Accuracy: Typical accuracy is ±0.01% of reading with up to 5 yr 

 between scheduled calibrations

19

Omega Engineering 

Signal conditioners for turbine 

 flow meters

FLSC-61 Operating temperature: –40 to 85 °C 

Input voltage: 12–28 VDC, 50 mA MAX 

Signal input: 

 Frequency: 0–10 kHz 

 Amplitude: 50 mV–35 V 

 Sine or square wave impedance: 10 kohms 

Analog output: 0 V @ 0 Hz, 5 or 10 V @ desired full-scale 

 frequency 

Full-scale range: 75 Hz–10 kHz selectable 

Response time: 95% of change in 1 s 

Linearity: 0.3% F/S 

Tempco: <2% of reading over entire temperature range 

Minimum load resistance: 250 ohms

19

Table 24. General instrumentation options for water-cooling system (Continued).
94

Item Model No. Specification Qty

Omega Engineering 

Miniature, quick-disconnect 

 thermocouples 

Probes at calorimeter inlets

KMQSS-125G-6 Type: K 

Sheath diameter: 0.125 in 

Sheath material: 304 SS 

Connector body: Rated to 220 °C 

Junction type: Grounded 

Length: 6 in 

Mating connector, cable clamp, and locking clip included

4

Omega Engineering 

Miniature, high-temperature, 

 quick-disconnect thermocouples 

Probes at calorimeter exit

SCASS-062G-6-SHX Type: K 

Sheath diameter: 0.062 in 

Sheath material: 304 SS 

Connector body: Ceramic, rated to 650 °C 

Junction type: Grounded 

Length: 6 in 

Mating connector, cable clamp, and locking clip included

17

Table 24. General instrumentation options for water-cooling system (Continued).
95

APPENDIX M—MASS AND VOLUMETRIC FLOW METERS FOR LOW-PRESSURE 

HELIUM/ARGON MIXTURES

M.1 Thermal Mass Flow-Type Meters

This thermal mass flow type of sensor has two elements—an RTD to measure the gas temperature and a heated element that maintains a constant temperature difference with respect to the flow temperature. As gas molecules remove heat from the heated element, power must be supplied to maintain 

a constant temperature difference. This power is directly proportional to the mass flow. The model 

10A thermal flow meter from Fox Thermal Instruments can be factory calibrated for a specific gas 

or gas mixture. All calibrations are NIST traceable. The instrument can be factory configured to provide 

a 4 - to 20-mA output that is proportional to either the mass flow or the internal thermal sensor’s actual 

voltage reading. The gas in the gas circulation system will be a He/Ar mixture. However, the exact mixture 

ratio will not be known until testing begins. (It may need to be adjusted to achieve the required heat pipe 

thermal coupling.) Therefore, the meter cannot be calibrated ahead of time for the exact gas mixture. The 

test chambers will include an RGA from which the actual gas mixture ratio can be obtained in real time. To 

solve the problem, it is proposed that the instrument would be configured to provide a 4- to 20-mA output 

proportional to the thermal sensor’s actual voltage reading, and have two calibrations performed—one for 

pure He and the other for pure Ar. The results are two calibration curves for converting voltage into mass 

flow as given by

m& He = f

He (VHe ) , (30)

and

m& Ar = fAr(VAr ) , (31)

where m& He and m& Ar are the mass flow rate, and VHe and VAr are the sensor voltage readings for He and 

Ar, respectively. Since the system will be operated at a pressure range of 50 to 100 torr, the two gas species 

will behave very much like ideal gases with virtually no interaction between them. Also, the thermal sensor’s reading is proportional to the amount of heat taken away by gas molecules impinging on the sensor 

as the gas flows past it. Consequently, each gas’s contribution to the sensor’s voltage reading is independent of the other and can be written as

V=VHe+VAr , (32)

where VHe and VAr are the contribution to the sensor’s voltage reading (V) for He and Ar, respectively. 

A mass ratio can be expressed as

m& He

m& Ar

= χHe ⋅ MHe

χ Ar ⋅ MAr

, (33)
96

where χHe and χAr are the mole fractions of He and Ar, respectively, and MHe and MAr are the molecular weights of He and Ar, respectively. The voltage (V) is obtained from the sensor reading, and χHe

and χAr are obtained from the RGA. With these four equations and four unknowns, it is straightforward to 

calculate m& He and m& Ar, with the total mass flow given by

m& = m& He + m& Ar .. (34)

For a sanity check, the instrument could be calibrated for a 50/50 He/Ar mixture and a checkout test performed to compare calculated results with the calibration curve. The cost of each calibration 

was quoted as $500 by the vendor.

M.2 Laminar Volumetric-Type Flow Meters

The alternate flow meter option to consider is a laminar volumetric-type flow meter that measures volumetric flow by using an internal restriction to create a pressure drop where laminar flow 

is established along the restriction. For laminar flow, the relationship between flow and pressure drop 

is linear and is described by the Poiseuille equation:

Q = (P1 − P2 )π ⋅r

4

8ηL , (35)

where

Q = volumetric flow rate

P1 = static pressure at the inlet

P2 = static pressure at the outlet

r = radius of the restriction

η = dynamic viscosity of the fluid

L = length of the restriction.

With known geometry, pressure drop, and gas viscosity, the device is able to calculate the volumetric flow rate. The gas viscosity is dependent on pressure and temperature. For the ranges of these 

meters, the viscosity changes very little with pressure and no corrections for pressure variations are 

required. However, changes in temperature do significantly affect viscosity. Internally, these devices 

measure the temperature and correct for variations in the viscosity. As a result, these devices can also 

provide a pressure and temperature reading in addition to the flow rate. The mass flow rate can be 

calculated from this information.

To use the laminar flow meter, a calibration—similar to that for the mass flow meter—relevant 

to the gas system He/Ar mixture ratios will need to be assessed. However, if the volumetric flow meter’s 

onboard signal processing provides signal outputs access for the internally measured pressure drop 

and temperature, this information along with the He/Ar mixture viscosity relationship can be use by 

the NI LabVIEW interface to construct the actual flow rate, without the need to calibrate the sensor for 

specific He/Ar mixtures.
97

APPENDIX N—TWO-BAND OPTICAL PYROMETER SPECIFICATIONS

Table 25 lists general specifications for two optical pyrometer units, available from manufacturers 

Mikron and Ircon. 

Table 25. Optical pyrometer specifications.

Mikron MI-SQ5 Ircon Modline 5R

Spectral region 0.70–1.15 μm; 0.97–1.15 μm 0.75–1.05 μm; 1–1.1 μm

Accuracy <2,732 °F±0.5% of the measured value in 

°F±3.6 °F

0.5% of reading, plus 2 °C @ 25 °C

Response time <10 ms, adjustable up to 10 s 10 ms, adjustable to 60 s

Resolution <1 K 1 K

Temperature dependence ±0.25 K per K deviation of ambient temperature Not listed

E-slope range (ε1/ ε2): 0.800 ... 1.250 0.800–1.200

Repeatability 0.2% of the measured value in °F±3.6 °F @ 25 °C 0.1% of full scale, plus 1 digit

Sighting Through-the-lens or laser (optional) Through-the-lens or laser (optional)

Analog output 4–20 mA or 0–20 mA 0–20 mA, 4–20 mA with 600 Ω max load 

(optional)

Digital interface RS 232 or RS 485 addressable, half duplex, 

baud rate up to 38.4 kBd

RS-485 (user selectable, 57.6K max)

Power supply 24 VDC±25%, stabilized, ripple <50 mV 24 VDC±5%, 8 W max

Operating temperature 32...158 °F at housing 0 to 55 °C, 32 to 130 °F (without cooling)

Weight 19.4 oz 3.1 lb

Spot size @ 13 in <0.25 in <0.25 in
98

APPENDIX O—VACUUM GAUGING SPECIFICATIONS

Tables 26 and 27 provide manufacture details for vacuum measurement components suitable 

for use on the test chambers supporting bake-out, gas conditioning/loading, and normal test operations. 

Table 26. Kurk J. Lesker multigauge box system with control boards.

Item Model No. Quantity

KJLC multigauge controller from Kurt J. Lesker VL8350302 1

Capacitance manometers sensor board × 2 VL6491301 1

Convection gauges sensor board × 4 VL6496301 1

Cold cathode gauge sensor board (option 1) × 1 VL6433301 2

Table 27. Vacuum pressure sensors for the multigauge box.

Item Model No. Specifications Quantity

MKS capacitance manomber 662A13TBD Range: 1 to 1,000 torr 

Resolution: 1% of indicated decade 

Accuracy: ±0.15% 

Operating temperature: 0–50 °C 

Connection: 8 (1⁄2-in) VCR®-F

2

MKS convection pirani sensor tube 103170012 Range: 10–3 to 1,000 torr 

Resolution: 1% of indicated decade 

Operating temperature: 0–50 °C 

Connection: 8 (1⁄2-in) VCR®-F

2

MKS cold cathode sensor tube 104210005 Range: 10–11 to 10–2 torr 

Resolution: 1% of indicated decade, 

 except 10% below 10–3 torr and 

 above 100 torr 

Operating temperature: 0–50 °C 

Connection: 8 (1⁄2-in) VCR®-F

2
99

APPENDIX P—DISTRIBUTED FIELDPOINT INPUT/OUTPUT COMPONENT 

SPECIFICATIONS

Table 28 provides a listing of general electronic hardware components needed to make up 

the real-time control system. Some of the quantities are currently to be determined (TBD), depending 

on the final test chamber configuration and hardware selection for other subsystems. 

Table 28. FieldPoint I/O components.

Item Model No. Quantity

Thermocouple module FP-TC-120 7

Analog input module FP-AI-110 TBD

Analog current output module FP-AO-200 TBD

Analog voltage output module FP-AO-210 TBD

Combination analog input/current output module FP-AIO-600 TBD

Combination analog input/voltage output module FP-AIO-610 TBD

Relay module FP-RLY-420 TBD

Relay module FP-RLY-422 TBD

Digital output module FP-DO-401 TBD

General-purpose terminal base FP-TB-1 TBD

Isothermal terminal base FP-TB-3 7

Four-port ethernet device server for RS-485 ENET-485/4 1

Four-port ethernet device server for RS-232 ENET-232/4 1

Ethernet network interface for FieldPoint FP-1601 1

Real-time controller FP-2010 1

TBD=Quantity will depend on the final system configuration.
100

REFERENCES

 1. Martin, J.J.; Reid, R.S.; and Bragg-Sitton, S.M.: “Design of Refractory Metal Life Test Heat Pipe and 

Calorimeter,” NASA/TP—2010–216435, 2010.

 2. “Standard Practice for Liquid Sodium Corrosion Testing of Metals and Alloys,” ASTM G68-80, 

ASTM International, West Conshohocken, PA, 1980.

 3. Deming, S.N.; and Morgan, S.L.: Experimental Design: A Chemometric Approach, Elsevier, 

New York, 1987. 

 4. ASME Boiler and Pressure Vessel Code, Section VIII, Division I, Section UG-28, American Society 

of Mechanical Engineers, 2001 Edition With 2003 Addenda.

 5. Dortwegt, R.: “Low-Conductivity Water Systems for Accelerators,” Proceedings of PAC-2003, 

Portland, OR, Vol. 1, pp. 630–634, 2003.

 6. Pellegrino, L.: “Experience With Long Term Operation with Demineralized Water Systems 

at DAΦNE,” Proceedings of EPAC 2004, Lucerne, Switzerland, pp. 2080–2082, 2004.

 7. ASTM International, “Standard Specification for Reagent Water,” ASTM D1193-99ε1, February 10, 

1999.

 8. Dow Chemical Company, “Product Information, DOWTHERM 4000, Inhibited Ethylene Glycolbased Heat Transfer Fluid,” <http://www.dow.com/heattrans>, November 2001.

 9. Dow Chemical Company, “HVAC Overview Guide, DOWHERM, DOWFROST, and DOWCAL, 

Heat Transfer Fluids,” <http://www.dow.com/heattrans>, May 1998.

10. Majumdar, A.K.; Bailey, J.W.; Schallhorn, P.A; and Steadman, T.E.: “Generalized Fluid System Simulation Program,” United States Patent No. 6748349, 2004. 

11. Verma, M.P.: “Steam Tables for Pure Water as an ActiveX Component in Visual Basic 6.0,” 

Computers & Geosciences, Vol. 29, pp. 1155–1163, 2003.

12. Furukawa, M.: “Empirical Expressions Selected as a Common Basis of Heat-Transfer and PressureDrop Calculations,” Transactions of the Japan Society for Aeronautical and Space Sciences, Vol. 36, 

No. 113, pp. 188–208, November 1993.

13. “Flow of Fluids Through Valves, Fittings and Pipe,” Technical Paper No. 410, Crane Company, 1988.

14. Incropera, F.P.; and De Witt, D.P.: Fundamentals of Heat and Mass Transfer, 2nd Ed., John Wiley & 

Sons, 1985.
101

15. Weston, G.F.: “Glow-Discharge Characteristics of Helium-Argon Mixtures,” British Journal of

Applied Physics, Vol. 11, pp. 305–306, July 1960.

16. Park, J.; Henins I.; Herrmann, H.; and Selwyn G.: “Gas Breakdown in an Atmospheric Pressure 

Radio-Frequency Capacitive Plasma Source,” Journal of Applied Physics, Vol. 89, No. 1, pp. 15–19, 

January 2001.

17. Loeb, L.B.: Fundamental Processes of Electrical Discharges in Gases, John Wiley and Sons, Inc., 

New York, 1939.

18. Goldstein, R.: “Radio Frequency Analysis Performed Using a General Concentric Tube/Inductor Layout,” Fluxtrol Inc., December 2004/January 2005.

19. Fox R.W.; and McDonald, A.T.: Introduction to Fluid Mechanics, 4th Ed., John Wiley & Sons, 1992.

20. Roth, A.: Vacuum Technology, 3rd Ed.—Updated and Enlarged, North-Holland, Elsevier, 1990.

21. Edwards, D.: “An Upper Bound to the Outgassing Rate of Metal Surfaces,” Journal of Vacuum

Science and Technology, Vol. 14, p. 1030, 1977.

22. Odaka, K.; Ishikawa, Y.; and Furuse, M.: “Effect of Baking Temperature and Air Exposure on the 

Outgassing Rate of 316L Stainless Steel,” Journal of Vacuum Science and Technology, Vol. A 5(5), 

pp. 2902–2906, September 1987.

23. Foust, O.J.: Sodium-NaK Engineering Handbook, Gordon and Breach Science Publishing, Ltd., 

New York, 1980.

24. Advanced Technology in Refractory Metal and Custom Fabrication, <www.rembar.com>, 

January 2005.
102

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Design of Refractory Metal Heat Pipe Life Test Environment

Chamber, Cooling System, and Radio Frequency Heating System

J.J. Martin, S.M. Bragg-Sitton, R.S. Reid, E.T. Stewart, 

and J.D. Davis

George C. Marshall Space Flight Center

Marshall Space Flight Center, AL 35812

National Aeronautics and Space Administration

Washington, DC 20546–0001

Unclassified-Unlimited

Subject Category 20

Availability: NASA CASI (443–757–5802)

Prepared by the Propulsion Systems Department, Engineering Directorate

M–1305

Technical Publication

NASA/TP—2011–216456

alkali metals, sodium, heat pipes, refractory metals, molybdenum, rhenium, radio-frequency heating, 

fission power systems, life testing

01–02–2011

UU 116

NASA

U U U

A series of 16 Mo-44.5%Re alloy/sodium heat pipes will be experimentally tested to examine heat pipe aging. To support this evaluation, 

an environmental test chamber and a number of auxiliary subsystems are required. These subsystems include radio frequency (RF) power 

supplies/inductive coils, recirculation water coolant loops, and chamber gas conditioning. The heat pipes will be grouped, based on like power 

and gas mixture requirements, into three clusters of five units each, configured in a pentagonal arrangement. The highest powered heat pipe will 

be tested separately. Test chamber atmospheric purity is targeted at <0.3 ppb oxygen at an approximate operating pressure of 76 torr (≈1.5 psia), 

maintained by active purification (oxygen level is comparable to a 10–6 torr environment). Treated water will be used in two independent cooling 

circuits to remove ≈85 kW. One circuit will service the RF hardware while the other will maintain the heat pipe calorimetry. Initial procedures 

for the startup and operation of support systems have been identified. Each of these subsystems is outfitted with a variety of instrumentation, 

integrated with distributed real-time controllers and computers. A local area network provides communication between all devices. This data and 

control network continuously monitors the health of the test hardware, providing warning indicators followed by automatic shutdown should 

potentially damaging conditions develop. During hardware construction, a number of checkout tests—many making use of stainless steel prototype heat pipes that are already fabricated—will be required to verify operation.

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